Well if the guy builds steel bikes from the ground up then he should probably know better about your specific issue since he was able to see, touch, feel and smell it in person. With us here at our keyboards, all we can do is look at a picture that gets grainy and unfocused when we try to zoom in.
Though being apprehensive, you should do what makes you feel good. Though based on his recommendation, I'd probably ride it. Though I would check on it often at first, and then as time goes by, probably check a little less often, but still check on it every now and then regardless.
This probably will have been better posted in the
Bicycle Mechanics sub-forum as I see more of the members that also hang out in the
Framebuilders sub-forum in there. I'm not certain if any of them have responded here, and I seldom see the ones I know build frames in this sub-forum.