I would kick the idiots ass. I mean really, if sheet was provided by me. I have high sense of property.
If he gave me sheet in the first place, and asks it back between sessions, then it would be more ok to me. My one dm re-used character sheets when we were poor students. Ripping would still be uncool, unless it is kinda social event on it's own, not just some whiny "hahhaha, you died"-thing.
i actually had dm who destroyed his monster/npc notes always when we killed it. He had versions of then on paper, he wasn't harming actual books. After he drew big X across the paper. It was sort of funny. We also had lists of monsters we had killled, and how many times.
Our kill-count was also proof that we had fought the monster and knew about it. We also added discovered special abilities of monster to that list.
I might keep copy of some details if dm does ripping thing of re-using thing. Just for record. So I recall what i used to play. And party names, which are usually listed on my sheet.
I've ripped my own sheet in couple of games.