If he is using an OE tire size, then he should be fine as the OE temporary size is designed to match OE fitment.  

If he's using a different tire size, there would be less damage to the diffs if the temporary spare is mounted in the front due to the open diff.  (Though less control under braking, so ... really not recommended.)

NO ONE should drive long on a temporary spare.  That's how they get their name.  "80km/h for 80 km" is the general guideline.