To the original question, what could she have done to better explain herself, the easiest answer is she could have asked them if she could spend another night because she was in pain, rather than having her BF do it for her. Alternatively, she could have better communicated to her BF how much pain she was in, since he was unaware of it and thus could not have properly communicated it to his parents.
The answer is NOTHING.
The parents had made their decision about the girl when they only reluctantly invited them into their home when they realized that their son would not attend without his g/f.
I think that if you aren't willing to give others even a little bit of room for understanding, then it's too much for you to expect they will do the same for you. You've told us over and over again how misunderstood you are and how awful it feels to be misunderstood, yet you aren't willing to give just a little bit to someone else that they may also be misunderstood?
If you're always assuming the worst of people, then that's all you'll ever get. Your relationships won't change.
You aren't reading the question. You can do what you can to try to ameliorate those feelings that the parents have, but the parents have made their own decisions as to the suitableness of the GF for their son. You cannot control THEIR actions, you can only control how YOU respond to them. THAT is how I deal with it myself.
I am tired of busting my butt to convince my b/f's mother otherwise. She is an 80 year old woman and her opinion of me is not going to change. Trying to convince her otherwise is only going to possibly cause ME damage, not her. So I do use my h/c placard despite her disapproval when we go out, I am polite to her because I love her son and I know that she wants what's best for him. He think that I am that person, so that is all *I* care about. So it is healthier for me to realize that there is nothing I can do to change her mind.
IMO, the g/f needs to step back and realize that there is very likely nothing she can do that will make her an optimal partner for their son in their eyes. That's not assuming the worst, that is being realistic in this situation.