I really do feel for Girlfriend, because that was a horrible situation--she walks in already knowing her boyfriend's parents are leaning against her, then has what is probably her one of her worst nightmares health-wise come true in their house, finds no accommodation, and has to drive herself home in pain.
A few months ago I was at a conference and spent too much time sitting (as one does), and had a horrible backache that prevented me from attending the last two days of the conference. I get backaches every couple of months but I'd never had one while traveling before and didn't have many resources on-hand besides OTC painkillers. Thank goodness I 1) had the hotel room to myself so I could deal with it in private; and 2) I've been working and attending conferences for a long time so my boss didn't have cause to be suspicious of me or think badly of me for not making the last two days. (Looking back, it could have actually been my gallbladder, which I recently had removed.) But, you know, what if it was my first conference with a boss, and I had to share a hotel room with someone (and fly for hours to get home)? That would have made my horrible situation 100x worse. So I definitely have some idea where Girlfriend is coming from.
I think like bah12 and others have said, it would definitely be worth it to me, as Girlfriend, to think about my own actions and what I had control over, and what I didn't. For example, even though I've had my gallbladder out now, I don't think that was the complete cause of my backaches, so whenever I travel I'm going to be bringing a few more tools like a heating pad and some kind of massager. And, I'm going to be more conscious of how long I sit for (because conference chairs are always cheap, uncomfortable ones) and get up and move around even more than I was doing, because obviously what I was doing before wasn't sufficient.
Those are things that *I* can control (what I bring, how much I move). If other people think badly of me for missing a session, or maybe I have a roommate who is completely unsympathetic about my pacing in the middle of the night (which I only do if I'm having a problem, not normally), that's not really something I can control. I mean, I can be apologetic and explain, and try not to bother people, but in the end analysis I'm not going to make myself be in pain to appease someone else. Say this unsympathetic roommate was my co-worker--yeah, I think that would negatively affect our relationship going forward. Hopefully we could still be professional at least.
But here there's additional complications--Girlfriend and the parents have a potential personal relationship as well, and Boyfriend still resides with them. For the sake of my relationship with Boyfriend, I might find it worth my while to reach out to the parents again and try to mend things, even though honestly I would feel like I shouldn't have to; but, as Vall suggested, I would also have a backup plan of staying in a hotel (probably just do that by default), and an emotional plan for how I was willing to proceed if the meeting went badly.
I think I would also need Boyfriend to show that he was meeting me halfway--moving out on his own would be a big step, for example. At least I would want to know that he had been thinking as deeply about the situation as I had, and was prepared to make some serious changes to prevent it from happening again, as far as possible. Basically I wouldn't want it to be only me trying to fix things for his parents/landlords--he would need to be working just as hard at it. Because it was no minor thing for Girlfriend to go through--this could easily break a relationship, I think, because you don't want to feel like you can't trust your partner to look out for you when you're vulnerable.