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.
There are 10 types of people. Those who know binary, and those who don't.