without proper education you are never going to know if you are doing something right. even with schooling, you still wont know unless you have some direction.
have your friend watch you, sure, but if your friend doesn't have the proper education, they also wont know whats up. so someone is going to have to pay some money somewhere.
you can train for years an get big, strong an look good. still doesn't mean you are doing it correctly. if you don't care about things like that, then ya, anyone can workout out.
I have quite a few clients who don't need a trainer. some have one for the reason you stated. they have already had enough instruction. others just like having a friend or someone to talk to.
after 3 months with a good trainer who knows what he's doing, a trainer is no longer "needed". wanted for w/e other reason is another story.