Similar situation here - my wife is using an old Mountain Bike on my Elite Direto that is fitted with a 11spd ultegra cassette. The mountain bike has an old 8 speed deore groupset. It does not work flawlessly, but there are 2 or 3 gears that do work, without chain rub.
So she’s not allowed to touch the shifters, but her workouts are in ERG mode anyway 
To be on the safe side (and not mess up the drivetrain of my road bike) I use a different part of the cassette, but this could be unnecessary.