As others have said, the argument doesn't work simply because he's responsible for whatever he takes. EVEN IF it's a tainted supplement, he's still fucked. The reason you're giving is fucking stupid man. 'Why would I even do it?' will NEVER EVER fly with the NCAA. Not a fucking chance.
Aside from this though, steroids actually do build muscle even when you're not working out. There was a study done with several groups, 2 of which were 'workout but no steroids' and 'no workout, but steroids'. The 'no workout, but steroids' group actually gained more lean muscle mass than the group that worked out but didn't take anything. Sooooooo your argument really falls flat based on that alone. It actually makes a ton of sense to take steroids if you have to spend some time not working out, in order to preserve muscle during that time.