Steam

Even worse, a VAC ban in your game will probably transfer to your account in general. You won't only be affected in that game, but in any games that check your VAC status.

I guess don't share it with them, or have a conversation about the consequences of their actions if they happen to cheat if you can trust them. Allowing for the loophole is worse than it possibly hurting a few people though. Cheaters ruin games for everyone else, and they don't have any control over it at all.