What the hell is the plan here, just flat out pick a fight, and take the prisoners and turn them in as if they caught them? The prisoners will absolutely out them so they would need to kill them and bring their bodies. But then the you will still get reported by the other party. And they have killed a tamed beast of a tamer, so they outright are attacking other adventurers. To make this work you would have to kill both the other party and the prisoners. Are they really willing to go that far? Mind you they could take the prisoners and make it a their word vs the other parties, but that is a huge risk, as if the guild trusts the other party more (if not use lie detection or other magic to see through them), and you do it to often you will ruin your entire career. So the only option really for pulling this is that they kill all, which picking a fight like that vs a larger party, with inhuman members is insane.
Actually, are they really even on the quest and have accepted it legitimately, they are traveling from town to town, are they on the run? And have they maybe just taken quest without actually accepting them.