Originally Posted by TennisCJC
I don't think a slightly closed face on the back swing is bad. Many pros do it. If she is exaggerating the motion and really closing the racket face, it could be a problem in my humble view - you don't want a lot of manipulation of the racket head in the swing to get it into a good contact position. My view is "excessive" movement just gives you more things that can go wrong under pressure. But, a motion that closes the racket face slightly is normal. Even some 1 HBH's do this.
Once more-the issue of grip is basic
BTW:OP does NOT say "slightly closed face"
See the video quoted above as well