I've read all the replies and I think I agree with a little of each idea. Because we are unlikely to ever hear from BF or the parents to understand their point of view, everything is conjecture so all I can do is think about what I would do, or would hope to do in the same situation and to me it all comes down to the lessen my mother taught me when I was little "always be gracious" and I think it applies to everyone in this story.
Parents - might not have understood what the heck was going on with "rude, quiet, lazy, disabled" (my guesses) girlfriend but if she was a guest in my home, knowing she would leave eventually, I would have said "of course she can stay another night. Does she need anything? A Dr? A heat pack? And then shown no frustration with having her but if I felt that strongly after she left, I would sit BF down and say "sorry son, but we found your GF difficult and would prefer not to entertain her again" depending on the agreement between BF and his parents about staying there.
GF - since the Op stated that both the GF and the BF were aware the xmas visit would be about more family time and not just their own time, and especially since the parents had to be convinced to have GF over, if I was GF i would have taken a small hostess gift, thanked parents for having me, discreetly moved around and taken meds as required, and when I couldn't get out of bed due to pain, asked to speak to mother quietly in private, apologise for any inconvenience, briefly explain the situation and give a guestimate of when I'd be able to leave. be an adult about it and not just stay silent and leave it up to BF (who sounds a bit clueless to me) to muddle his way through talking to his parents.