Using Inept! in response so a class ability

[Norse]

Hello!

Player A who is a wizard is in a combat. He announces "I'm going to charm the monster so I get the treasure!"

Player B says "No you're not!" and plays the curse "Inept" on player A.

Is it legal for player B to play this curse in response to another player using a class ability or does Inept! have to be played prior to another player trying to use a class ability?

Thank you!