If you make a commitment to someone to be monogomous, that's no longer "friends with benifits" that's pretty close to being a relationship sans the feelings. But if you had a mutual arrangement to sleep with only eachother, for reasons not concerning emotions, I would say it's not necessarily cheating in the sense that their betraying anothers feelings, but its a betrayal in the sense that they had an agreement and this person broke that. Whoever this friend is, I'd advise them to talk to the person they have the arrangement with and tell them they don't want it to be monogomous if they're going to sleep with other people. A deal is a deal, but whoever had the idea to make it monogomous, probably had a relationship in mind. But no, since it is not a real relationship, I would not consider it "cheating", but for health purposes the other person needs to be aware that the person that they're sleeping with is messing around.