I'm totally fine with a wizard/sorcerer, etc. either knowing or making a roll to know how to end a standard polymorph spell like that, then wasting their action doing something, or directing another PC to do so. The polymorphed fighter could certainly try to move towards his allies, no problem there. But it requires the allies to actually make a stabbing thrust etc. Heck, they still have to make an ATTACK roll to see if they even hit. Even if the fighter stands still, he still has an AC that someone has to hit (unless said sorcerer or wiz uses magic missile obviously).
5th/6th level PC's in a standard point buy game are going to be, what +7 to attack? So they miss on a 1,2, or 3 against the toad's AC. Then someone else has to try, etc. Either way I'm with the, the character needs to have knowledge of why he'd suddenly want to kill/attack his friend before I'd let that fly at my table.