What is the best way to handle your 17 month old being hit/pinched by their 18 month old friend?

They were best friends until about a week ago and now her friend keeps trying to hurt her - bending her fingers back is another one. It isnt over toys or anything else, and my child doesnt even retaliate, just gets upset if it has hurt her. The little girl's mum, my friend, tells her off but it just happens again 5 minutes later.