Melbourne's reputation is that it's more theoretical and Monash is more practical, which one of these appeals most? Look at the course guides of both unis. hat subjects would he be doing at both? Which seem more interesting?
Ranking isn't really something to worry about. Firstly, it's in Engineering, so he'll get a job regardless and secondly, if he's looking to go international, 'Monash' and 'Melbourne' probably won't mean all that much. MIT means something, 'random Australian university', not so much. If he wants to go on exchange though, then that is worth looking at.
If he doesn't want to do a double degree then he definitely shouldn't. It is the same workload as a normal degree, it just takes longer. Maybe he should think about honours instead (an extra year on the end).
With the scholarship to Monash, I think that sounds better. 1 hour out of Clayton isn't so far to travel also.