This is long - sorry, and I hope it makes sense.
Well, we had a good chat last night and GS is certainly glad to have a change of scene for a couple of days. I won't dwell on that because things have moved on today.
This morning, GS and DH were out playing golf when I got a call from the GF. She was very emotional and desperately wanted to talk to my GS but his mobile was off (on the kitchen table next to me). When I said he was out she asked if she could talk to me about the situation. I said she could certainly talk to me, but I wasn't promising that I would pass anything on to my GS, or even tell him that she had called, because I didn't want him to be even more upset.
There is far more going on than I realised, or than she had told my GS, and we talked for more than an hour.
She comes from a reasonably well-heeled family, father is a "self-made man" and pompously proud of it (I've met her parents twice), but she is the first in the family to go to Uni. What I didn't know, although my GS did, is that her parents are paying for her to go to Uni, rather than her taking out a student loan. As a result, they are putting her under massive pressure to get a good degree.
At her parents' persuasion, this year she is living in a hall of residence that is miles from the main campus. The big idea seems to have been to restrict her access to nightlife on campus. Most of the students in the hall are postgrads, mature students and overseas students who work all hours and don't socialise. She says that some of them hardly seem to leave their rooms except to go to lectures and cook a meal. Many of the older students have cars, but she has to use public transport and that dries up at around 10pm each evening.
After the first week of term, during which she panicked about my GS for the first time, she was so miserable that she pretty well moved in with him because he lives much nearer the campus, and they were spending almost all their time together except when they were in lectures. Although she has loved being with him 24/7 over the summer, she realised that now she wasn't getting any work done. She was feeling incredibly guilty, and felt that she had to pull back from the relationship to do what her parents wanted.
She felt she couldn't tell my GS all of this because they had said "her degree should come first and he will only get in the way of that". Well, of course her degree should come first, but that doesn't mean that she has to spend a year alone in Outer Mongolia! 
She is a conscientious girl who would try to work hard wherever she lived, but when the two of them were crammed into his single room, she just didn't have the space, mentally or physically, to be able to focus on her work.
She is terribly upset about the hurt she has caused my GS, anxious about her final year, under pressure from home and a bit like a rabbit trapped in headlights - she can't see anything clearly. In a nutshell, her parents were guilt-tripping her last term, and have continued to do so ever since my GS and she returned from abroad in September. The implication has always been that if she doesn't do everything the way they want, they will pull the plug financially.
We talked about various possible solutions, but in the end it came down to getting her out of the miserable hall and into a more sensible place where she can be a student who takes personal responsibility for herself.
She and my GS had a very long conversation this afternoon. She explained everything properly to him and he will go with her to the accommodation office on Monday to find out if she can move. We're hoping that there might be a room going spare because of people dropping out of their courses since the start of term.
She has called her pushy, controlling parents and told them (not asked them) that she is moving if she possibly can. I have also spoken to her mum and we have discussed the reality of student life, depression and even (brave moment!) the effects of parental pressure.
My GS says he obviously knew that she wasn't happy in the hall, but he doesn't seem to have twigged how much stress the work situation was causing her (he is the type who does it all at the last minute and still gets annoyingly good results), and he certainly had no idea how much grief she was getting from her parents.
He hasn't said that they should get back together yet, because he knows it must be one step at a time, but I think the mood is now "cautiously optimistic". I have warned him not to let his protective instincts towards her to get the better of him.
If anyone is still faintly interested in this rambling saga, I will update on Monday! Thank you all for your help.