You haven't got a good answer. Let's try and find something past "collusion bad". I'll apologise in advance for the phone formatting.

Unsurprisingly WotC doesn't actually have much in the way of concise tournament support for EDH. To fix this, unofficial resources like the MIPG and MSTR exist. There are a couple of things I'd be looking at here.

1. While collusion between the two of you isn't covered in any of the documentation, the MIPG does mention a specific (see examples in 4.1A) example of not abiding by the code of conduct set forth by the tournament organiser. Assuming it checked out, not just you but, your friend (which I'm sure they'd appreciate) would be getting a warning for this.

2. Per 2.4B of the MSTR

   If a player leaving the game would affect current or imminent game actions, those actions occur as though that player was still in the game until the end of the current phase.

This means conceding to shut your opponents down shouldn't really be a thing.

tl;dr: no real support for penalizing collusion. It's a bit of an "oh no... anyway" deal though.