Re: What are your thoughts
The bottom line here is the skill of your proposed welder. Mending fence posts and making good structually sound welds on a bicycle frame are two very different things.
I'd want some examples of his work on thinner material than steel posts.
What kind of equipment does he use? Stick? MIG? (with shielding gas or flux core?) TIG?
Do some checking. A structural failure of a frame could get you hurt.
Age and Treachery Will Always Triumph
Over Youth and Skill & "Charlie Don't Ride"