In our local group rides there is a guy who rides in the fast group on a mountain bike. That is always my reminder that money is best spent on something like trainer road rather than upgrading your components to save grams.
In our local group road ride there is a guy who rides a single speed gravel bike that he assembled himself out of random used parts, and he still beats the rest of us even on hill climbs.
A good 80% of the people I see riding 3k+ road bikes (in an area that likes to bike, there are lots of them) are carrying far more extra pounds on their bodies than they could ever shave off the bike. It's probably safe to say that the vast majority of people riding bikes have far better bang-for-buck performance improvements available to them than a new bike or components, but don't choose them....
If he's riding in a "fast group" then the weight savings will be worthwhile. You can do this with a used 20 year old aluminum frame. It doesn't take elite hardware to benefit from light weight.
If he's keeping up with the fast group already, he may not care (i.e. it's not worthwhile, to him)
I'm not saying you have to spend a lot of money. I'm saying people mostly don't do the obvious performance changes anyway, and for most riders it doesn't really matter (as they aren't doing anything competitive enough to need it).
The weight has to be way less important than the tires. Mountain Bike tires have more ground contact area and have very aggressive tread compared to a road bike tire, which is going to have a huge effect on efficiency. You can hear the difference if you ride each style of bike over the same piece of pavement.
I stuck some smooth tires on the 30 year old mountain bike I was riding around town. It's amazing what a difference it was. It wasn't that expensive or hard to do either.
Of course it made me run headlong into the fact that the bike was geared to go up hills and I was hitting the top gear of it even on relatively easy rides.
