Minor spoilers for anyone who hasn't played the Dragonborn add on.

With that out of the way, NPC follower Teldryn Sero has the "adept destruction" perk. This allows you to enchant his gear with the fortify destruction enhancement to reduce his destruction spell costs by 100%. Moreover, this grants him infinite charges with enchanted weapons that use the destruction school.

I've tested this pretty thoroughly, and his enchanted sword seems to show no signs of charge depletion. He's the only NPC in the game with a listed perk that reduces his spell costs. This has made him my new favorite follower. Have fun. :3

Edit: Odd. restating the game made this not work. Though he killed limitless enemies with it until I quit to desktop and reloaded. C'est la vie.

Couldn't you put fortify destruction on any follower?

No, generally enchantments that fortify skills don't work on followers.