I find nothing wrong with just being upfront, honest with the person whether she is a very close friend or just an acquaintance or co-worker. As a matter of fact, it should be easier to be perfectly honest with a friend. It is not necessary to tell her your feelings about her work ethic. Just be honest about the direction you wish to head in. If the situation were reversed, would you want your friend to continue doing something she was absolutely sick of? Do you think your friend would want this for you? Just do not make it about her. Make it about you.
Your artform should be a joyful pursuit and if chairing the shows is no longer fulfilling, it is time to move on. Your pal just might surprise you and feel the same way. And, if the worse happens, and she is upset because you are moving in another direction, that needs to be her problem, not yours.
Hope this hasn't sounded too preachy but I've been there, done that and have found honesty is, indeed, the best policy.
Best wishes, let us know how it goes.