+1 on wheels and tyres being more important than a couple of hundred grammes in a frame.
Same with fork – a better functioning for a light rider over saving a few grammes.
Another +1 on wheels/tyres/ weight saving (but agree that the lighter frame > heavier frame if everything else is equal). Using that Marley linked to earlier shows even with ‘decent’ spec bikes there’s plenty of weight saving opportunities: going tubeless from inner tubes saved 300g per wheel, rear tyre a further -250g, also a faster-rolling Ardent in place of the Minion DHR2. Rim swap saved 160g per rim. Handlebar another 150g or so. But obviously if starting from a frame-only it’s easier to spec what you want.
I’ve not had the Marley stripped to bare frame to know the weight, wish I had.
But very much this:
The most important thing is that he’s having fun