You shouldn't date your friend's ex.
Henry  is going to a party.
Henry wants to bring a fun date.
Henry invites Maria, who he met at the coffee shop.
Henry decides to invite his best friend's ex, Lulu.
Maria fits in great with all of Henry's friends, and becomes part of their social circle.
Henry's best friend is upset when he sees Henry and Lulu together, and never speaks to either of them again.