Aaaaah Mike, and I thought you were going to be talking about your hands!!
Liz's bike had crap levers and cables on for a while which were very hard to move, but with LX levers and new cables they are silky smooth again, so XT stuff should be fine. How long have the cables been on for? And have they been lubed at all during that time? Might be worth looking at fully enclosed cables…
The other option is a gripshift, but then she's obviously not getting on with that either!
How free is the front mech? Does it move smoothly?
Oh, and fancy getting out riding soon?