Nothing on the hind end but tearout and redo. All the work was on the front end in product and labor selection. Better hope he knows how to properly waterproof a shower. Most of the budget hacks are clueless.

https://www.ceramictilefoundation.org/homeowners-guide-to-hiring-qualified-tile-installer

Hire better next time, or this time, if this doesn’t have to be torn out and done over.

Thanks for comment. He says he doesn't know how to fix it...

That’s is because there is no fix for doing it wrong but a complete redo.

Common error. The first heat cable should be 17" from the wall to locate just in front of the vanity toe kick. Any further and you will have warm heels and cold toes. Not a pleasant way to begin every day. Complete redo on the contractors dime. " I thought I hired a professional who knew what he was doing. " You did not. Refer to the link Sophie provided.

He screwed up and put the heaters too far from the front of the vanity.

It cannot be fixed, it can only be torn out and replaced. The tiles and the heating elements will probably be ruined so you are looking at a complete remove and replace.

I think he owes you a whole new floor, with the warmers in the correct places for user comfort.