Supposedly it is $400 minimum. I say supposedly because they don't really ask for proof as you just have to agree to their rules. Some companies don't follow the rules exactly as they should be though. Some companies (and I won't mention Lincoln Heritage...oops!), will add a lead balance in order to get it up to $400.
In general, the rules are, you owe someone money, you need to pay it. You don't, you get a vector listing. But I assume you know this, so what is different in your friends case that this shouldn't apply?