Come on, nobody actually WANTS to stand for 4 hours in an uncomfortable, hot, moving vehicle. Anybody would be tired and sore at the end of that, regardless of your age, or the clothing/shoes you're wearing, what hand luggage you have to hold, if you're already sleepy/were hoping to sleep on the journey etc. When you're not there and actually being asked to, it's easy to take the high road and claim you would have given up your seat (i don't travel on trains so no idea if you pay for the privilege of reserving, but surely if you do you don't happily give money away free anywhere else in life so willingly) so to say you're disappointed/ashamed OP's daughter kept her seat is a bit rich, especially without knowing every last detail of the circumstances.
I'm only 28, look physically healthy enough, but i have back, neck and knee issues due to a car crash a year ago, plus i have an unstable bladder, which gives symptoms matching those of urge incontinence if i am standing for even a relatively short length of time, completely without warning. I'd be in significant distress standing for even 5 minutes and risking wetting myself, let alone 4 hours, it would be physically impossible for me. This woman, and those suspected of giving dirty looks, could not have known whether or not your daughter was physically healthy and without disability (nor expect her to declare it to strangers when putting her on the spot if she did) and should never have targeted her directly but asked a more general question to the carriage.
Your daughter clearly told her no, that she had reserved her seat and wasn't willing to give it up, at which point the lady should have had the grace to ask someone else/the carriage as a whole. Her making a judgement without knowing your daughter that she needed the seat less and again pressing her to give up her seat, was rude, and could even have come across as aggressive. I can understand your daughter feeling embarrassed and awkward by this point, and trying to diffuse the tension by putting in her earphones, even if she doesn't suffer from any form of social anxiety, so that wasn't necessarily a rude response either.
I don't agree with previous posters that she should have gone to find the guard, or offered to help look for him etc. By moving out of her seat, what is to say the lady wouldn't immediately have sat down and refused to move, or that someone else wouldn't come along and sit in it while she was gone, and refuse to move? I'm assuming even the guard cannot physically remove somebody from the seat, even if it is a reserved one they have sat in, nor is there a guarantee of a seat elsewhere or the invitation to sit in first class without cost as an alternative. She probably just wanted to be left alone and protect herself and her reserved seat, get where she was going and not be hassled.
Ideally the lady should have reserved her own seat, or gotten there earlier to secure an unreserved or priority one. Even if she was traveling last minute, the latter option should still have been open to her. She took the chance and it didn't pay off, by all means politely ask a general group if anyone can offer a seat, but don't expect one, single anyone out, or refuse to take an initial polite refusal for an answer. As for the other passengers, well, none of them jumped up to offer did they, so they are hypocrites if they are judging her for not giving her seat up.