She should tell the truth, as being caught in a lie can have serious consequences, including being barred from the US.
The other thing she needs to do is have her story straight, and lots of evidence to back it up, as to why she will go home after 3 months and not stay illegally in the US.
This includes how she can take three months off from whatever she normally does back home.
travel.stackexchange.com/users/264/DJClayworth
I've had my (now wife) visit me several times.
I'm American, she is Irish.
She visited me at least 5 times, if not more, before we got married on official fiancé visa.
Sometimes I was with her coming over, sometimes she was alone.
Sometimes she was greeted with kindness, and others interrogated with malice (I was with her that time :/).
It's a coin-flip as to which your girlfriend will have.
These were all 90 day visits, same as you.
While I'd also suggest the truth as others here, I'll also throw in that she doesn't have to disclose all the details.
They might just raise more questions that are irrelevant to her coming for a visit.
As far as border folks are concerned, you are a US Citizen working in the US.
Period.
Mentioning recent relocation and all that will just prompt more questions.
They're looking at her and her intentions on coming over.
Will she take any American jobs, get on welfare, bring in extra family members, terrorism, etc.
In one of our visits, my girlfriend was taken into the little room, needlessly interrogated about marriage and future immigration which ended with the agent throwing the passport back at her saying "get out of here before I change my mind".
That situation prompted the getting officially married idea.
On that point, make sure she doesn't mention anything about getting married or being your fiancé; whether it's true or not.
You should definitely not plan on getting married this vacation, but anything said that might even hint at the possibility of an un-approved immigration or anything like that will cause delay and more questions.
Your relationship status should be private beyond that it actually exists.
So, to sum up: she is just your girlfriend.
She will be visiting you, an average working American Citizen for 3 months.
No plans for marriage, just having some fun spending money in America.
She has a return ticket booked for 90 days from then.
It's true info, and doesn't raise further questions to complicate the entry process.
Most border folks are grand, but often will start asking further when you mention irrelevant details.
Once, I messed up and mentioned a bottle of Absinthe I was brining back
(it was legal).
They had to go get my luggage and inspect the bottle while I waited for 45 mins only for them to say it was ok.
I should've just kept my mouth shut.
travel.stackexchange.com/users/39500/coblr
When I was a student I had an American girlfriend and my conversation lasted exactly 2 minutes every single time, so my first bit of advice is don't panic.
The questions I was asked were: Reason for visiting.
How long I was staying.
How was I able to get ninety days off work and how did I intend to fund my stay.
I answered truthfully that I was visiting my girlfriend, I would be staying for ninety days and as a student I didn't need to return to my studies until September.
I told them that as I was staying with her family my living expenses were quite low
but I had X amount available and a credit card.
They then waved me through without a backwards glance.
I should add that I entered the US three times like this in the summer, once in the spring and twice at Christmas over a four year span and never had a single issue.
There will always be a lot of horror stories
but when you read them they mostly come from oversharing.
I sometimes feel that border control is one of those occasions in society that us introverts get ahead!
travel.stackexchange.com/users/30874/Dustybin80
At CBP, always speak the truth, because lies could bring anyone toward critical circumstances, if she don't have any wrong intention, she can explain her purpose of visit, sometimes CBP officers are nice too.
Good luck!
travel.stackexchange.com/users/61339/Nestsouls
As noted by DJClayworth and others, the border agents are very concerned that her trip will not turn into an illegal stay.
For an example of a similar situation going poorly, and the reasons for it, see: http://www.news.com.au/travel/travel-updates/incidents/molly-hill-reveals-how-dream-trip-to-hawaii-turned-into-a-nightmare/news-story/b3833259e7ffe3e130dbf73c852f2f82
travel.stackexchange.com/users/61502/Toen
Some personal experience and a checklist of things you will want to have on hand (in order of importance).
I had a girlfriend visit me from out-of-country many times
and this was always what they were most interested in.
Proof you will leave the country.
Have copies of your plane tickets home or proof of purchase.
Carry cash or other funds on you (with proof if possible).
Try to have enough to cover your stay.
Know where you are staying, full address and who lives there (note:
a roommate who was not mentioned answering the phone and does not know is a big red flag).
Tell the truth, if they catch you in a lie it will be much worse.
Don't bring anything inappropriate across the border (as it might give them more reason to refuse entry).
Make sure your stories are straight and if possible be available for a call from the border agent.
They will sometimes call to confirm the story.
You don't need to, but cleaning out your phone/laptop of anything incriminating might be a good idea (as they can request to review it and read your messages/etc).
travel.stackexchange.com/users/37098/Sh4d0wsPlyr
I'm an American citizen and will soon relocate to the US for work. I currently work in Italy.  My girlfriend will come in September and stay 3 months with me.  What is the best thing for her to say to the border patrol when she comes over?
What should my girlfriend say to border patrol when visiting the US?
travel.stackexchange.com
<us-visa-waiver-program>
travel.stackexchange.com/questions/93743
travel.stackexchange.com/users/61490/Cheesnut
She should tell the truth, and be ready to explain how she can take this time off from her normal job and provide details of a return flight, but she does not have to volunteer all of the details without being asked, including about the relationship. In particular, she should avoid mentioning anything about getting married. Telling the truth is the best way to get through the border, but it is ideal to have proof of your return trip as well aas keeping yourself from oversharring especially if the topic is marriage. 
Telling the truth at the boreder is the quickest and easiest way to get through. Just don't be too over sharing as it could be problematic Keep information about relationships to a minimum especially if it involves marriage. Then it may be best to avoid to topic completely having proof of your return to your home country makes things a lot smoother