PC Master Race

This is a silly question, but if the card(s) came with screw-on antennas, have those been attached?

By chance did you check his wifi cards in your PC? The chances both are bad are low, but you never know.

Have you tried moving the pc itself. I had a similar issue at my old place where the pc was sat in a corner with the antennas at the back. This probably had a negative effect on the signal to my router. Only when moving and shifting stuff around my desk did I figure out that the pcs position was the issue.

First thing I'd check is the frequency the wifi is running at. Right click the wifi icon and do Network and Internet Settings. At the top under properties it should say 2.4, 5, or 6ghz.

If it's 2.4 are you in an apartment where there's a lot of people? 2.4ghz is basically unusable in most apartments for anything other than basic stuff.

If it's 5ghz then that's good, but you'd want to make sure the signal is strong and the link speed is decent. Is the wifi icon in the task bar full? Or is it missing 1 or 2 lines? Anything other than "full" according to that graph is going to get bad, but shouldn't be 5-10% packet loss. For the speed click on where it says wifi in that settings screen, then hardware properties and look at Aggregrated Link Speed. Is that more than like 100?

Z87 is 4th gen Haswell on Windows 11 which isn't 100% supported, so that could be the issue too. Can you try a live USB of Linux to see if it's a software issue or hardware?

Perhaps?

Have you tried moving his system closer? Have you tried using your system near his location? It's not just microwaves that can interfere with wifi, but also some kinds of fluorescent lights, or even infrastructure (pipes, electrical, mesh retaining, etc).

What is the signal db loss? Ping is a catch-all metric but isn't reliable for nailing down specific problems. Also the varying ping times is called jitter, and some jitter is perfectly normal - wifi tends to amplify ping and jitter. Don't get too hung up on high ping unless it is a problem.

If db loss is acceptable, is his tcp/ip stack compromised? Run a virus check, ensure his network configurations aren't being hijacked (check dns, proxy, etc). Reset the stack and configs if necessary.

Has his router been compromised? There is firmware that is going around hijacking routers for botnets - very hard to identify and reverse from what I recall. Try another router if you have one. Aside: Might be a good idea to factory reset the router and start with a clean slate too.

Is your wireless network congested? too many devices can cause large ping and jitter, even if they aren't on your network. Removing devices, changing wifi channels, disabling guest connectivity, and enabling game mode on the router can improve the congestion issue to varying degrees.

TPM has nothing to do with networking, which is good because there are bootable USB OS's you can run to test your hardware without wiping your current install if you feel like it might be another type of compatibility issue (TPM would make this much harder to do).

How did you test ping? Was it just in games? Or did you actually ping various websites? Does ping improve if you are hardwired to the router? What is the router model? Is QoS on? Have you run traceroute to see where the packets may be getting dropped?

Try a linux live ISO so you can rule out software

You need to hardwire the thing to properly start troubleshooting. Otherwise you may be scrambling around in the wrong space.

You might check the PCIe pins for any signs of corrosion and/or hit them with the good old isopropyl+toothbrush treatment.