Ask him if you can borrow his car to take on the race track for a few laps at 120 mph. If he says "no" he probably isn't as good of a friend to you, as you are to him so stop renting out your plane. Jokes aside, if you're not comfortable with his flying style, just stop renting your plane to him. I wouldn't rent out my plane to anyone - hell even when I'm on the plane with another pilot and they ask for controls I take over if they bank my plane more than 15 degrees for no good reason. My plane, my rules. If you're concerned about him being weird afterwards, just tell him "the group of owners" decided you can no longer rent the plane or some insurance excuse - no renters allowed anymore. I'm sure he can find a flight school or some other place to rent planes if he really wants to go flying.