I've read somewhere that it is a good thing to do things like playing a musical instrument if you have dementia as it helps to keep the brain active.
My OH used to play classical guitar years ago and I've now got him back onto trying to play again. The thing I find best is not to put any pressure on him but to let him go at his own pace, being encouraging and supportive. And also to reassure him that he needs to learn from the beginning to get familiar with it again and it's normal not to be able to pick up from where he left off.
It's going very slowly but I do leave it up to him. If he hasn't done anything for a few days then I try and bring it gently into the conversation and if he doesn't want to do it, then I let it lie.
Sorry posted this while you posted your last comment. It may still be worth thinking about a one-off "lesson" for someone to come and play the piano with her but warn them she may not feel up to it when they arrive