The idle circuit in a Holley is richer than the mains.
He has to read the plugs first to see if they are white or black.
He is only presuming that the engine is lean. He hasn't said it is backfiring. Running out of fuel, lean, even at part throttle will generally backfire through the exhausts at some point of throttle opening.
I say this thinking that the car was running well with that carb on it. IF he just installed the carb, then you are likely correct, that it needs to be set up. Agreed on that also.
He's closer to you than me. Have him bring the car around. I'm sure you will get it straightened out for him?
You know sometimes there isn't anything wrong with the carb at all, it's just a bit of a mismatch out of the box.
Holley on an aluminum intake in the cold weather really are lousy running setups. They just freeze up to easily.
I was just running my 68 GT350. It's 30 degrees and the automatic choke is still being a bear. Warmed it up. Than it sat for 30 minutes and the engine cooled off so fast had to do it all over again. GRRRRR!
They just don't like anything much under 35 F.