He most probably doesnt want to marry her though she may want to and think he does. Shes 15..hes 15? and at that age (well not only that age, all ages) theres a lot of temptation and she will have been swayed by his sweet talk. you may have read those jokes about how a man speaks to a woman and she thinks he wants to marry her. Theres some truth in those jokes, a woman can easily be flattered.
Are you sure your parents are separating because of her? There must be more to it than that.