If he has a FoS out he's not shooting, and therefore being useless.
And it takes about three seconds to kill a heavy with a medic on him, assuming you are the only person attacking him, which you probably are not.
No offense, but if you don't know how much damage a sniper can do just quick scoping headshots, which is damn easy to do against a class like heavy, don't act like you do know. Heavy is by far the easiest class to deal with as sniper if you have decent aim, and I actually prefer the medic to be pocketing the heavy rather than other classes just because I know I can kill the heavy or force him back in a matter of seconds very reliably.
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