I learned not to ever let anyone ride as a passenger with me. I dated a guy once who gave a guy a ride home from his brothers party to prevent a fight, (5 minutes away) and the guy passed out on the back of the bike during a turn and didn't lean. It caused him to go off the road and a guide wire for a light pole took his leg off above the knee as he went by. He passed away a few years ago from diabetes.
I don't think I will ever trust anyone enough to let them ride on the back of a bike with me after that.