If it's just those 2 heads up in a hand against each other, it's not really collusion, it doesn't change anything.
IF they do this with other people in the hand , then yes, you could call it collusion.
Unfortunately you run into this alot at casinos
, especially in smaller poker rooms. Your bio says you're from Melbourne so I know your poker room their is massive, but I play at crown perth and I can tell you this happens almost always on a daily basis, but only if they're 2 friends heads up with each other.
However they should have probably just checked it down and then winner takes pot.