Maybe he has a spinning disk in it? I see lots of laptops in that price range that go for the 1TB HDD instead of the 256GB SSD. Windows 10 on a 5400RPM HDD is pretty miserable these days, the OS just can't stop touching the disk and it's forever IO bound.
I've had the same problem. I had a i7 T420 with 16GB of RAM that was lightning quick when I got it and over a few years became completely unusable even for basic tasks. When I hit the Windows key to open the start menu, I could turn and take a sip of coffee before it opened. Reformatting and reinstalling from scratch did nothing to improve the speed.
I switched to a Macbook soon after and it is just as fast today as it was when I got it four years ago. I recently fired up that old T420 and popped an older, unused SSD into it. Instant game-changer. It is unbelievable how much faster it is with an SSD, the same speed it was with Windows 7 when I first got it.
Hitting the Windows key on Windows 7 even with an HDD was instant. Hitting the Windows key on Windows 10 with an HDD tok seconds. Installing an SSD brought it back to being instant.
Any idea what's wrong with it?