I sure he’s got grx800 1x I don’t know if like sram you can mod them and fit a shifter paddle in there or not. So I’m guessing you need a new lever? He’s not very DIY.
Personally I say just go fixed ;D
Do they specific 1x shifters that are cheaper?
Personally I'd pay the extra to have a more versatile bike.
I keep the SS/fixed for shopping bike duties
So what are they using for the other lever?
If they're only riding flat stuff why not ditch the RD and go SS? Do it properly :)