Can you prove undue influence if a person was competent?

Family member was giving "friend" money and we believe she felt under pressure/influence to give him the money. She is now incompetent to act on her behalf due to a stroke. After she became ill we discovered that she was giving him money when we wrote bills for her. Saw entries in checkbook ledger, etc. and have had others to tell us she gave him more cash. We are told he has her lock box with money and important documents in it. My SIL lived at apartment complex where this person lives and had told us he kept her upset, etc. but we didn't know she was giving him money.