I speak from own experience as I once had a frozen shoulder myself. This is something where you need to work with a physical therapist. The reasons for getting a frozen shoulder can vary. There are cases when frozen shoulders all of a sudden “melt”, but there are often underlying problems which may even require surgery.
That does not mean that the client cannot exercise at all, even for the upper body. In all likelihood, she cannot raise her arm and will employ a lot of compensation strategies to get her arm / hand up somehow. Try to avoid reinforcing those compensations. Of course, no upper body exercise when she is in pain.
The main thing is to get your input from a PT. It is a tough one to deal with.