Maybe it is, as TurtleDove says, a "life stage thing," and Rachel's friends simply don't have a lot of experience dealing with a person their age who has physical limitations. However, that does *not* excuse their behavior.
One of my dearest friends has rheumatoid arthritis and some other issues that compromise her mobility and wear her out easily. She uses a walker, one crutch, two crutches or nothing, depending on what kind of day/hour/minute she's having. We have roadtripped to a couple of concerts together, and I wouldn't have dreamed of leaving her side for a moment (except to use the restroom) any more than I would consider hopping a spaceship to Mars. We are both in a different "life stage" from TurtleDove, and from Rachel and her friends -- we're both 55. But illness and disability can strike anyone at any age.
TD, I know from your other posts on this forum that you are in excellent physical condition, and it's a point of pride for you. As well it should be -- you work very hard at it. But someday you will not be, even if you are spared a disabling illness or accident. It's called getting older. When -- not if -- that happens, I hope you will gain some appreciation for those who understand and work with/around your limitations.
As for Rachel, I think her friends are simply in denial about her limitations. She's well enough to go to the concert, so she's well enough to keep up with them, go partying afterward, etc. I admit to being guilty of this with friends who have mental health issues. You're on meds, right? So everything's cool, right? Except it isn't.
If I were Rachel, and these friends came calling again with another concert/night out invitation, I'd say no and spell out chapter and verse why: "When we went to the show on X date, I couldn't keep up with you and I got flak for it. You left the venue without me to go to the bar and left me to find my own way. This was unpleasant and downright scary. I don't want to go through that again."