Tony, If you utilize a 2P 100A breaker in the main panel to feed the sub panel, that's not really a tap, just a feeder circuit. You don't necessarily need a main CB in the sub panel, but it might bu useful for local isolation.
Two things to make sure of:
1. Use adequately sized feeder wire.
2. As noted above, someone should do a load calc on the entire electrical service and distribution system.
I presume your friend is getting a permit for the addition? Did he get the electrical permit also? Most building authorities will want to see this info too.