It's not necessarily the nut, but it could definitely be if he put in taller frets without replacing the nut! A new nut (and a level) should be part of a refret. I wouldn't give the guitar back to you buzzing. You can use guitar strings as feeler gauges to check the string height at the 1st fret. The lowest I like to go that won't buzz when played hard is .016" - .020" 1st string to 6th string. Ime, you can go lower than that if you want really effortless playing but it will buzz if played hard.
However, I would have returned it to the luthier/tech... they knew they were putting on larger frets and shouldn't have given you a guitar that buzzed.
By the way - PLEASE tell me you are playing early Metallica/Megadeth on that?!?!?!?!?!