You may be able to use the rack on your car (depending on what rack system you have) to support the bike while you spin the pedals and shift.

Before I got a repair stand, I could set my bike rack up on my deck railing. Enabled me to do a few repairs this way, but the low height made it hard on my back to do anything too involved.

But since this problem only shows up in the middle ring, and she has said she's had the chain come off a few times, I'd check for a worn or bent sprocket on the middle ring. Sight down the middle ring while slowly turning the pedals and make sure that every tooth is aligned.