Is it possible for you to talk with the therapist before the appointment? You could let him know what has been going on. If not, maybe you could encourage your Mom to make a list of things she would like to discuss with him, or would she let you send a note with her to the appointment? Here's a link that does say that Medicare covers therapy:
Does Medicare Cover Mental Health Care? Good luck and give your Mom a BIG kiss for me. It is soooo cool that you are so concerned with her happiness!