Bear in mind that for bedroom players, 5 watts is plenty loud. Deafeningly so.
So you don't need an enormous amp probably. I built a Fender 5f1 champ circuit amp. I'd probably build a slightly larger amp next time. Not for the volume btu for the features.
You can always turn a 100W amp down to practice in a bedroom
No, not really. A 100w amp won't distort at low volumes.
If the guy wants to gig he should build a bigger amp however, 40watts would be about right. Huge amps aren't needed as much anymore as most halls have PA systems.
a 100W transistor amp