Is he using a TAP or THREAD CLEANER? A complete set is well worth the price. They have them at Summit Racing.
I'm no pro but have worked with metal for some time and it has always been a no no to run a tap through existing threads. Always use a Thread Cleaner (Chaser) tap or make one.
You can use an older worn out tap as a good thread chaser. A new tap can be used but you have be be careful because you don't want the remove material, just clean them. Greasing the tap actually helps a lot.