You only need a few inches so go to the big box and buy a foot of THHN/THWN of the same gauge as the two offending wires. The #12 stripped from the type NM cable would probably be fine but why take the chance of failing another inspection?

All flexible conduit is a pain to make look nice.

I'd let him go ahead with whatever he wants to use in the garage.

The boxes need to be secured and the cables need to be secured within a certain distance of the box as well as throughout the run. How far from the box depends upon if they are clamped in the box or not. Clamped needs to be secured within 12 inches and unclamped within 4 inches as I recall, I could be wrong about the latter. Where accessible the cable needs to be fastened every 54 inches (four and a half feet) I think. Cable lead through drilled holes is considered fastened for the length of the run.

 

NM without a clamp needs to be secured within 8".