This is literally one of the textbook examples of collusion.
Temporary trading of players, specifically in bye weeks, is the #1 example of collusion by most peoples standards. It is banned in every single league I have ever played. Cause otherwise, no team has to account for bye weeks ever. Which is a pretty normal and important part of fantasy.
Imagine you’re in a league where players just trade their studs back and forth to fill out bye weeks. Does that sound like a league you’d be interested in playing in?
EDIT: There is no way to prove intent of the trade at the time unless you’re a mind reader. All you can do is look at what happened after its happened and it has resulted in a temporary trade to elude the bye week. Even if they did plan it, which it looks like they did, they can just deny it and sell the BS story you’re saying and there’d be no way to prove it. That’s why most leagues just straight out ban temporary trades.