[ipgyst]

Does your friend have security-cam and hole-cam recordings to review? That's the easy way.

Statistics is the other way. The hard way. Something like "What are the odds that these 3 players would never play against each other in a pot in this tournament?" Let's say an uber stat geek comes up with a probability of the event as .01 (getting this number is the hard part). That means the event will happen 1 time in 100. Not a big deal. If the same event happens again in the next tournament though, that's odds of 1 in 10000, which makes a high probability of being collusion. Or the stat geek might get a probability of the initial event as .000001, or 1 in a million. That's very likely collusion as well.