"You have a guy behind a tree and he moves right from tree and fires an arrow and then ducks back to just move left and fire another arrow." He says that is terrain mod, I am like, NO , that is cover bonus.
I think I might see where he's trying to come from.
We can assume if he is not firing, he's using the tree for cover. I'd consider a tree full cover, so there's nothing to dodge so long as you don't expose yourself. If you want to fire at your foe, then you are coming out from cover to fire your bow so now you have only partial cover for the round (however much you want assign, I'd say 50%-75% depending on how generous you want to be).
So, using a tree for partial cover and attacking your foe: Hiding behind a tree, and popping out to fire... at which point the foe (who is likely holding on you) fires at you, and you dodge back behind the tree, only then to pop back out and fire back. That's not a terrain bonus, that's dodging. So if he specifically has a dodge skill, and wants to dedicate action to using the dodge skill, I might allow that. Now it's a question of if you have or want to allow such a skill. I don't believe here is one in RMSS (there's tumbling evasion, but you must dedicate the entire round to it). In my opinion his Qu, and therefore DB, are dodging.