If this was a few years ago I would say it would be no problem. But with covid and parts being difficult to obtain it might be a bit of a headache and more of a over the winter project. Just a 105 group set if you can find one is going to be around $600. Then you have to buy forks, wheels, seat /seat post, headset etc. He's going to quickly exceed his budget.
Maybe it's where I live but I find a lot of decently priced smaller sized road bikes on facebook market place. But you got to be fast and always watching. When one pops up if you sit and think about it for a hour or two it will be gone.