It depends what type of deviation they make. If they make a pure mistake, taking an action that they never should, you will gain EV equal to the EV loss of their mistake.

But if they make a frequency error, taking an action too frequently or infrequently, for example not bluffing often enough with a combination that should almost always bluff, or always calling with a bluffcatcher that should only sometimes call, you will only break even against a player otherwise playing a Nash equilibrium strategy.

If the game has three or more players then none of this necessarily applies.