Protection against an aboleths enslave ability

I am structuring the first part of my campaign and have decided the big bad is an aboleth. However supporting the aboleth will be a wizard, who has sort the creature out and is willing to work for it in return for the knowledge and power it can give him.

What I am looking for is a way for this NPC to protect himself from the aboleths ability to enslave (he has researched and read about them and is prepared). This is his insurance to ensure his pact with the aboleth is 2 way.

What magic items or spells available to a wizard would allow this protection?

In game he is living in a town 5-6 days travel from the aboleth and visits it only when needed to discuss progress and the next stages of the plan, or take individuals to be enslaved.

A magic item would be preferable as it would allow the players to gain an advantage for when they finally find and face the aboleth itself.

The players will be interacting with this wizard from level 1-6 initially as an ally but I am happy making him a higher level enemy and so giving him high level spells if appropriate, I can have him avoid direct confrontation and instead try and escape. Or the players come up with interesting ways to try and defeat him that don’t require direct combat if necessary if he is too high a CR to face directly.