My 6700 XT is relaxing at 1080p in most games so buying a 6800 XT solves his 1080p problems for a LONG time as 6800 XT is way more capable. As peeps mentioned above, buying new PSU is a must, I don't trust his PSU. Go for something like Chieftec 750 W with 80+ silver shield on it, must be enough for a 6800 XT. But I truly recommend lowering GPU core clocks by 200 MHz so the card won't spike the PSU out in their entire lifetimes.
6800 XT will also allow for going 1440p monitors with 100ish refresh rates without a doubt. FSR doesn't look awful at 1440p, unlike its terrible performance at 1080p, so your mate will probably not notice 1080 to 1440p switch performance-wise but catches a little more beaut in the image quality if he stares from close enough (PPI * D >= 4000, where PPI is Pixels Per Inch in his display, and D is his eyes' distance from that display in inches).
Oh by the way, buying an "overkill" card is not a bad idea at all. Relaxing card will live longer than that performing at its max still not allowing for comfortable framerate. Lower noise and heat ratios will come in handy, too.
CPU-wise, my only answer is WAIT TILL 7800X3D-BASED PLATFORM BECOMES AFFORDABLE. Buying any higher tier AM4 CPU is a waste of effort.