There is a re-tapping tool that cuts through the metal and cuts new threads. This also means you need a new axle, and you need to do both sides to accommodate the new axle.
You can get re-tapping tools at many decent hardware stores. The big box stores may not be the ideal place, but they are a good starting point.
My recommendation:
Contact Landon Balk for this service. Also, what yoyo is it? Not all yoyos can be re-tapped.
If the threads have just minor damage you can buy a tap of the right size and just run it in by hand to straighten them out. If the damage is major you need to find someone with a lathe that can properly set things up to drill out the hub and tap it for the next larger thread size, Then get an axle to match.
I’d still recommend he send his yoyo to Landon Balk, who lives in FLORIDA. It would be pricey.
It comes down to “cost to do the service, assuming it can be done” vs “cost of trying it yourself and possibly screwing it up and having to replace it if the job gets messed up” vs “just get a new one”.