From what I understand from your post, your daughter has not been able to get back into regular training since her last injury and has since expressed the wish to quit? If that's the case, I would not let her quit with her final experience being an injury and lack of confidence.
What I've told gymnasts (and their parents) who told me they wanted to quit (usually I had noticed this moment coming up for a while), is the following:
"Yes, you can quit but it would be nice if you could finish on a high note. Come to practice for another month or two, try to give your best, and then, on a set date, you can quit. If you start to feel differently during this time, you can change your mind, but we don't expect you to and you don't have to explain yourself further."
This has always worked really well and I've seen kids equally go both ways - some quit and some decided to continue and others decided to continue in a less demanding program. All of them seemed to regain their passion - even the ones who ended up quitting. Of course, I try to make it a little more fun for them during this time. I'm less hard on them and try to boost their confidence a little more and explicitly focus on what they can do.
You'd definitely have to talk to her coach(es) though....it's important for them to be on board with this!