Dinghy boarding ladder
One trick we learned fast was to leave your fins on. A few agressive thrusts with the fins, coupled with the arms on the dinghy side (tubes or gunwale) and we had no trouble.
Alternatively, you can rig a stout line (knotted is best) from the opposite gunwale, across the top, then down into the water with a bowline loop for a foothold. Rigged at the right height, your wife can insert her (unfinned) foot, grab the knotted line, and hoist herself aboard. It might take someone else on the side opposite of your wife to prevent the dinghy from flipping over on her.
I''m sure you''ll figure something out. Good luck.