Wait a minute. Is this a 220v welder? And does your generator put out 220-240v? If its a 120v genset, i don't see how you can make this work.
6-50 220v should need 2 legs of 110 and a ground. No neutral. 14-50 has 2 legs of 110 and a neutal.
Sounds like his generator has 14-50 which is 240V.
Welder 6-50 is also 240V but no neutral . 6-50 is from old days where for example 240V ranges didn't have electronics requiring 120V so it has no neutral. Only 2 hots (240V) and ground.
14-50 has a neutral to get 120V. That would be the W in the above wiring diagram.
Based on the above everything he needs is there. I am just not sure if he can buy an adaptor or not ... or he should be able to make a cable with no W (neutral).Disclaimer:
It's been a while since I worked on these kind of stuff and have forgotten and need to refresh my memory. If I get additional info or have time to research, I will update.
Meanwhile please double and triple check and do some research or talk to knowledgeable people who work on these on a daily basis. I don't want to give you a bad info.