As PapaBear and Fresh Wind have suggested - have her record the next session....we all have recording devices on us at all times - Use a smart-phone, or borrow someone elses and put the video cam on during the next session.
Do it covertly or in the open.
Covertly will get his actions recorded.
Out in the Open method will curtail anything that may be happening.
I vote for Covertly.
Any other action she or her mother does will be a “He said-She said” scenario.
Regarding recording: First check the law in your state about covert recording of a conversation without consent of all parties to the conversation.