Now, the spell says that you keep your personality, but, you have the intelligence of your new form. Can the PC's actually achieve their goal using polymorph or not? 100% DM interpretation.
If the PC polymorphs the fighter into a T-Rex (or a giant ape), will the PC attack allies? Why not? It now has the brain the size of a walnut. Basically, it's a shark on legs. Why would it distinguish friend from foe?
Again, 100% DM adjudication. Not impossible, of course, but, if I was 14 years old again, and a new DM, I can imagine that that sort of thing gets handled ... less than perfectly the first time around.