Your friend is the kind of asshole who gives pit bulls and other breeds a bad name. They should not be allowed to own dogs. First, you can decide if you want to deal with the side affects/risk of AAS and decide not to use them. The dog can't. secondly the only reason to do this would be to make the dog bigger and more agressive. I work in an animal hospital. I get tired of seeing all the damage done by irresponsible pet owners. Picture a little old lady, her husband dead, her kids dead or moved away. The only companionship she has is her dog. I have seen them come in to my hospital in shock and crying because they were walking their dog down the road and it was attacked and killed by a dog that was lose or that got away from the owner. These people are old and know they will die in a few years so most of the time they don't get another dog because they don't want to leave it behind. They just spend their last years alone.
If your friend does not care about any of that remind him of what happened to those people in NY. If his dog attacks someone and they find out he gave it juice he is going to jail for a long time.