One thought: could it be that WH has been hitting on the grad student, who THOUGHT she could manage the situation to her advantage and really DIDN'T sleep with him? And now it's all getting out of control for her?
I know from my own and friends' experiences in both academic settings and the military that when a boss hits on a younger woman, sometimes she "makes nice" but controls the situation as best as she can and doesn't let anything happen, knowing it's a short-term situation she has to get through and not wanting to risk losing support from the authority figure who is looking out for her. Sometimes saying no only makes the man more determined and more likely to try to be her Knight in Shining Armor (you said she wasn't doing well before he becamse her mentor?) Could he actually be chasing after a woman who is really, in fact, using him? It could even be that she's a "nice girl" who feels trapped and knows she's leaving soon. Not SURE this is the case, of course, but thinking back on my life and the lives of some of my friends over the years at university and in the Pentagon... This sort of thing DOES happen.
If so, he's going to be pretty upset when she backs way off after graduation to get away from him when she doesn't need him anymore. Sure, she'll be polite enough to get recommendations, but as she gets established and gets her first job, she will probably want to get on with her life with a younger guy without the baggage of kids and an (understandably upset) ex-wife.
Discovered OW1: 1/10, H refused to talk. H moved in w 1st OW: 3/10 H cheated on OW1: 12/10 H left OW1, moved in w OW2: 4/11 D: 9/11 Still miss H. Don't understand, H just left, never even talked w me.