he will almost certainly lose all of these gains once off (this is true right?)
You said you did this same cycle...how did things end up for you post cycle? If he loses all of his gains, he's too clueless to be using anyway. Sounds like he's got his mind made up...although I would probably advise against it too at this point.
juicing with only 6 months of training is not a good idea for several reasons, firstly lack of experience.. IMO you need at least 2 years of training experience to fully understand you body in the gym, secondly lower cost-benefit ratio.. as he could probably still gain a lot more naturally, so why take all the additional risks of steroids if you don't really need them to achieve your goals, and thirdly greater chance of injury.. as his body is still relatively new to lifting and joints and tendons won't be a strong as someone that has a few years lifting under their belt