My son's girlfriend says she is bisexual and he is affraid of her leaving him for another woman

My son says his girlfriend says she is bisexual and he is always worried she will leave him for another woman. I try to tell him that is not the case so it alleviates his worries. Should he be worried? They are in a long distance relationship where they are always on line together, she comes to visit him and vice versa while he is in his last year of college. My son is very faithful and don't even look at other women and she don't look at other men and they are always face timing etc. Once he is out of school they plan to get engaged.