Help! My PCs polymorphed my boss enemy! What do I do?

Our Druid has recently gotten access to the polymorph spell and has started polymorphing the opponents I so lovingly crafted for our (well, mostly my) enjoyment. (So far, he has been turning them into rabbits, not yet killer whales.)

Now, the canonical response to a polymorph is to reduce the polymorphed creature to 0 HPs, upon which it will transform back. Or to attempt to break the spellcaster’s concentration.

However, polymorph is a 4th level spell, not a cantrip. It does not seem all that likely the target’s allies know the spell, can connect the dots when their friend suddenly turns into a bunny, and start to attack the bunny, or the spellcaster. It seems more likely that they will just blink upon seeing their friend transformed into a bunny, shrug, write him off, and continue battling the PCs as before. Which would essentially take the target out of the fight.

I have been thinking about allowing the target’s allies an Arcana check each, against some reasonable DC (e.g., 14, which is 10 plus the spell level of polymorph). On a success, they recall hearing about the Polymorph spell and about the two possible remedies. On a failure, they don’t. In which case the target will remain polymorphed, until it gets hit by a stray fireball or some such. I might even allow the target itself an Arcana check, and on a success, it will try reducing its own HPs. Well, this might at least make for some fun.

It looks to me like polymorph might be a bit overpowered against opponents whose allies only consist of non-spellcasters, because they will likely not know what steps to take to break the effect. Is there a better way to role-play what happens to a polymorphed enemy?