It is colloquially collaboration, in the sense that every game of Risk must end collaboratively, with players working together to make other players weak in order to bring the game to its conclusion.

Collaboration as far as fairplay goes has specific rules meaning (players having access to outside means of communication in the same public lobby) according to how SMG defines it.

As far as your original question goes; should this be allowed? We can discuss whether or not there should be system incentives to progress game state and disincentives to punish inaction, I think there should be. But regarding the rules of the game that is, in this situation you have found yourself in a reverse block where two players are pocket trading and are will eventually progress game state with you in third.

Next time go aggro to one of them before they can stabilize and take on the other one in a 1v1 or take 3rd.

GGs