I'm gonna take the middle road here. I've played a lot of DoNs over the years and collusion is rampant in these games. However, there are different kinds of collusion; some expected and implicit to the format of the game and others that seemingly make little/no mathematical sense and aren't really collusion at all.
Expected/implicit collusion is when all the mid and large stacks "gang-up" on the shorty at the bubble to get him out so that all remaining players cash. Goes with the format, and is expected though never voiced.
What you're talking about here is not that. His play makes no sense, BUT that doesn't necessarily make it collusion, either. It's far more likely, imo, that for whatever reason you, specifically, were his target. Maybe you pissed him off earlier, maybe he didn't like your avatar, maybe he drew straws before match and picked you. Who knows? Anyway, it seems like he wanted you out regardless and was willing to let the other player live in order to achieve this goal.
While, mathmatically and financially, his play makes no sense; that doesn't automatically make it collusion either. He is free to play any hand in any manner he wishes. It only becomes collusion if he had a prior arrangement with the all-in player that he would fold to him at the end, rather than look him up. Or, some similar agreement, prior to this hand, that they would work co-operatively regardless if either of them became short. Something like that.
Bottom line is you can't collude with yourself. If he just let this guy live to get get you out because he had targeted you that's a douchy move for sure; but it's not evidence of collusion. To collude the other player would have to be actively working with him due to a prior arrangement where they would soft-play each other even if the math said not to.
Without the active participation of the all-in short-stack; there is no collusion. If you suspect there was such an arrangement; you're gonna need lots of proof. You're gonna have to have many, many games that they have played together where they participate in a strategy/move similar to this in order to spare each other. Not saying it doesn't/couldn't happen, but you're gonna need LOTS and LOTS of proof over many, many games.