I think I would go along with Freedom -- steer your friend in another direction , like vacuuming, dusting, doing the bed linens and towels. I think it's easier than having to tell her straight out you don't want her help in the kitchen.. I don't think I could do it myself, I would be so afraid of offending and maybe losing a friend of 30 yrs.
Maybe she could even do some errands, whatever she does, impress on her what a help it is to you.