S
Sunseeker
Guest
My personal take on it: I'd let them do it once, but once a monster is smacked with the first sneak attack, I'd personally rule that they don't get anymore sneak attack damage until they change locations, say at least 10 feet or so. Hiding behind the fighter is perfectly reasonable. I wouldn't even give disadvantage on the roll. But just because you're behind the fighter and I can't see you doesn't mean I don't know you're there, and I don't know you're about to jump back out and shoot me again. Sneak attack dice represents striking an unsuspecting target from an unknown location. That doesn't cut the mustard for me.
If he wanted to do that and then, roll unseen behind a bush 15 feet away off to the monster's flank, then take a shot from the new (unknown) position, I'd probably allow that. But munchkin-ing the rules so you're just ducking back and forth behind the fighter with a short bow getting sneak attack dice every time? Nope. I wouldn't allow that.
By the rules, they'd still get sneak attack with a bow, even if not hidden as long as an ally was engaging the enemy.