If running a FT technology business is anything like the client support I provide on the side - the technoogy will be the easy part.
Managing customer expectations is the biggie. Knowing how to deal with a customer, even the angry one who makes no sense at all, is critical. Getting blamed for things that "worked before I brought it to you" when they aren't related, is pretty common.
Being held responsible for a part that fails simply because you installed it 6 months ago, is pretty common.
Anyway - it's much more about the customer interface than it ever is about the tech. If you (or your friend) are at all questioning your technical abilities, that's a sure sign you haven't pushed the needle on your learning gauge. Get out there and absorb some more obscure info!
I'm pretty confident in my knowledge, it's just that I haven't done a lot of actual work with that knowledge(I've built two computers and set up a smoothwall computer).