I think he will die cuz fishman normally live in the water and with a devil fruit he will drown, of he will be one of the devil fruit users which can swim
There is actually a Merman who ate a devil fruit. Vander Decken IX. He lost the ability to swim like anyone else who eats a Devil Fruit. Which sucks because he's a merman. Now he has to wear a bubble suit when he's in the water.