When you say your mate "brewed in them" do you mean he fermented in them or dispensed from them ?
Either way, and legalities aside, it would be more expensive and much more of a hassle to "brew" in a CUB keg than it would be to go and buy a carboy. What are they ? About $30 ? With the keg you've gotta get it plasma cut / welded and modified. Unless you had the gear and the nouse to do it yourself. But if you did, you probably wouldn't be asking us.