The rulebook states this about activating mutations on page 69:

When you activate a mutation, no one can stop you and you never risk failure. 

Another description in the Mutations section on page 69 says:

Your mutations are your trump cards, superhuman and unstoppable powers wired into your very DNA.

In terms of Conflict

In a conflict (Chapter 6), activating a mutation usually counts as an action.

And in the side table for misfires:

The mutation gets supercharged. You get back the MP you / just spent, and you can immediately – in the same turn – activate the same mutation again, against the same target or another.

So our conundrum is "when can a player activate their mutation"? Is it only when it is their turn during conflict if it isn't reactive, or can it be at any time and the player consumes their next action?
The specific scenario we had earlier was where an enemy was approaching PC1, and PC2 said they activated their human magnet mutation to push the metal on the enemy npc, forcing the npc off balance. Our GM and the player in charge of PC2 were going tit for tat about what it means for "When you activate a mutation, no one can stop you and you never risk failure." 
Is the "no one can stop you" a true "trump card" that can be used to change the actions? Or as the rules allude to for conflict or a misfire, can the mutation ability only be used during the players turn in the conflict?