What is his rights regarding the power of attorney that he has given his sister?

My best friend had a stroke 2 years ago. At the time he was in bad shape and we all thought he would not make it. During that period, his sister (only relative) received his power of attorney. Since then, he has made a very swift recovery. He only has speak impairment (somewhat) but can still vocalize and talk, just takes a little while to get it out. He has no physical impairment or mental impairment. He wants to move from his sister’s house because she makes his life a nightmare. She has power of attorney and tells him that he can't move or she will put him in a home. Can he move (keep in mind she also has control of his money). What can he do?