+1 But they do that because they don't understand the physics involved. All the experts forget they were not born bicycle or motorcycle riders. It's a learned skill. So is being a passenger.Best thing to tell them is to sit still, don't try to correct the lean angle just go with it..
Start out slow. Don't scare the n00b. I remember so many guys enjoying scaring the GIB (Girl In Back) and then yelling at them because they squirmed. Move slow and corner slow.
As the OP's wife hasn't made any major faux pas getting them hurt I'd say head for a big open industrial parking lot on a Sunday. No concrete chocks just smooth asphalt. Ride from point A to point B in a more or less straight line but doing shallow gentle weaves almost but not like cone practice. Then try large circles and graduate to large figure 8s. On a nice day in a wide open space I'd probably do this without (yes without) helmets so you can easily communicate your intentions. Tighten things up a little at a time and pickup speed a little at it time.
Having your wife lean against you reduces the likelihood of involuntary 'assistance' until she gets her 'bike balance'. Bumper helmets may be an issue. A backrest would be a big help.
The quality of ability of a pillion to 'passenger' is a product of the quality of the rider's ability to ride. You need to build and instill trust and confidence.
Anybody who can't do that shouldn't endanger another by letting them be a passenger and doesn't deserve a one anyway.