My wife, bless her heart, never understood how to ride as a passenger. When I’d take a corner, she’d lean the opposite way.
It's caused by counter steering. Next time ya go into a sweeper notice which hand pushes, and which one pulls. You actually turn the wheel right to go left, that's what makes you lean, riding on a smaller area back and off the center of the front wheel.
Bungee chord her to you. Works great.
Whoops! Did you ever drop her from the bike? Before I married my wife, she rode on my motorcycle as a passenger. One day I went off road on a mountain, went up over a ridge and hit a rut in the path. The motorcycle stopped cold at the rut, I slammed into the steering bar, but she kept going over my body and tumbled down a hillside. She had mild scratches and bruises, but was pissed. Yelled at me to take her home, and never rode on my motorcycle again - ever. We've been married almost 50 years, and she's still pissed about that motorcycle ride.
Same problem, solved it by just taking my girlfriend and left the wife home.