Polymorph Potion on Mate card

[JNL32]

I was playing the original Munchkin with my wife and I used a Mate card on her monster which makes a copy of the monster. My wife then played Polymorph potion on top of the original Monster to take the treasures, and was going to fight the Mate Card.

My question is, if she played Polymorph Potion on the original Monster, wouldn't the mate card by nullified, and could she play a Polymorph Potion on a Mate card?

Hope my question make sense.