J.S.
Biting is common at this age, not necessarily of a divorce (issues over that will probably surface later)
18 month olds do not have the communication skills to express wants, needs and feelings. They also think everything is theirs. Since they can't say the right words, often they will bite, hit, kick, etc. In a day care center it is multiplied because of a group of children doing the same thing and the biting can become reactionary as well.
Have your friend try to increase the vocab, almost constantly talking to her. Also, check for teething which can give her the urge to bite.