Convert Welder to AC Gen Set?

No simple ones. A large and expensive inverter would be required, or a rewire of the inverter internals to get the right output voltage and regulation (perhaps possible but an additional transformer would likely be required as the one in the welder is NOT rated for continuous operation or the output voltage you require).

I would NOT recommend it unless you know what you are doing around high current and mains voltage, and have a full circuit diagram of the unit in question due to the level of project difficulty and safety issues.

If he burns the exciter out the first shot at power he will have a lot of problems.

I can't think of any cost effective way to do this other than removing the welding generator and substituting it with an AC generator head. I have a 300 AMP gas drive Hobart and I've been down this road before. I'm just thankful that the Hobart has a 4kw 120VAC output. 20kw generator heads can be had very reasonably.

No. Needs to operate at either 1800 RPM with 4 pole alternator (hard to find,$$$$) or 3600 RPM with 2 pole alternator.

Cheaper to buy a military surplus unit and tune it up. How big do you need--how many KW.

well, if you wanted to retain the ability to weld with it, you could consider getting a AC generator head and a DC motor and close couple them and drive the DC motor with the welder output. It probably would cost more than getting a mil-surplus one but it would work.....