As a speech therapist I always prefer to defer to a physical therapist or orthopedist to make the decision about safety and desirability, but from the standpoint of making beautiful music, it’s great for morale, listening skills, general finger/hand dexterity, problem solving, visual motor skill, social interaction and probably a lot more things I’ll think of after I post.
If she’s benefitting from piano, your PT or orthopedist may feel that harp would just add another helpful dimension.
If she likes the idea, let her school know what she’s doing so that she can participate more fully in discussions about hobbies and interests in school (sometimes 8 year olds get so wrapped up in conversations that they forget they have something good to contribute too).
Of course you’re going to want to make sure that what she’s doing isn’t physically or any other way stressful, but I’m sure you’d be doing that anyway.
Good luck with this wonderful activity!