my friend is probably close.

the problem is that if he were to enter a contest, he'd diet down to get leaner, and doing so lose all his mass.
it is not the amount of muscle that causes us to think someone is not natural, it is the amount of muscle while being in contest conditioning.
if everyone were to enter contests with the type of conditioning in the pic above, amateur bodybuilding would be more popular, healthier, and people would be more willing to believe it when someone claims to be natural.
but everyone tries to get down to 3-5% bodyfat, and you're not going to do that and keep your muscle mass without drugs.