I'm going to purposely use the credit card which I know has the highest merchant fees
What is the basis for your thinking? Why do you want to use that credit card?
It is not my card so the merchant fees were going to be paid by someone I don't like much.
It is not good to have somebody have to pay more just cause you don't like them, revenge can lead to people trying to get back at you. It can sometimes never end.
I don't care what the merchant fees are. In fact, the higher the better since this person will have to pay all of them.
That sounds very illegal. Most people won't pay attention to the details so much just to get revenge, and people may think you're acting very strange and unlikeable.