My stoker is handicapped and most
definitely *would* have an issue with bars that moved. She cannot
ride by herself; this is why we tandem.

I'd still suggest trying the bike in its normal configuration. My
experience with stokers who could not ride bikes has generally been
that they are good at not influencing the bike's balance (as opposed to
many cyclists who lean in anticipation of turns when first riding as a
stoker). As long as your friend doesn't actively oppose your steering
efforts you should be able to easily control the bike. Could she hold
the front handlebars in a position closer to the stem than usual? That
would reduce the effects of any force she applies to the bars and
minimize the motion she would feel when you steer around a turn.