If there's already too much pressure on her hands, a longer stem isn't going to help that. And if she's complaining about pressure on her hands, it sounds like upper body position probably isn't the issue with her climbing.
Rustle up a different wheelset to try. Sounds like she has a shimano drivetrain and you said you didn't have another microspline wheelset, but a wheelset with a sram cassette will work well enough to gauge climbing improvements. It's not gonna shift perfectly, but it'll give an idea of whether weight reduction and skinnier tires solve the problem.
Bookmarks