Cannot boot or recover MacOS (even from USB installer drive or internet) after Boot Camp installation

Hi all - I have searched the forum but (so far) cannot find any answers to this particular problem.

My friend's teenage son installed Boot Camp on his 2019 MacBook Air. Since doing so, he has been unable to boot back into MacOS. He gave it to me to have a look, and here's what I found:

1- Windows boots fine, but there are two Windows 10 installations to choose from on startup. One works and launches the Windows 10 desktop, the other produces (Windows-based) error messages and offers to fix the corrupted drive instead. From the Windows desktop that does work, I noted that there is no D: drive (no MacOS partition), and there is no "Boot Camp" app in the Task Bar ^ menu. I did find the Boot Camp app under "Program Files" and ran it. The Boot Camp control panel showed up, and I could select MacOS from there.

But here's where it gets interesting:

2- When booting up into MacOS, I get a momentary spinning circle (dots) after which the computer reboots - this cycle keeps repeating itself to no end.

3- When booting the MacBook while pressing Option/Alt, I *do* see two partitions: "Macintosh HD" and "Windows". If I choose Macintosh HD, I get the spinning circle/reboot cycle.

4- If I restart the Mac while pressing command-R to get to the recovery drive, I again just get the spinning circle/reboot cycle. I cannot boot into the Recovery partition.

5- I created a bootable USB drive (using Monterrey) and plugged it into the MacBook. I then started up the Mac pressing Option/Alt, and I saw all three options ("Macintosh HD", "Windows", and the Monterrey startup disk). When I select the USB startup disk, it restarts, and I get the momentary spinning circle/reboot cycle again. I cannot boot the Mac using an external USB startup disk.

6- I started up the Mac while pressing Command-Option-R to get to the Internet Recovery. The spinning globe appeared and asked me for the Wifi name and password. I entered these and the normal progress bar appeared with the "This may take a while..." message. Once the progress bar completed ~15 min later, I did not see the menu; instead, the system rebooted and the same spinning globe appeared asking again for the WiFi name/password - and this cycle again kept repeating. I tried this in two different locations (two different Wifi routers), and got the same result. As such, I cannot access Internet Recovery from this Mac!

7- Lastly, I turned the Mac on while pressing the T key to get into Target Disk mode. The USB and FireWire icons appeared on the screen floating around as normal, but the Mac is not recognizable when I plug it into 2 different Macs that I tried (it is like I never connected anything; no HD icon, no nothing). Placing the Mac in Target Disk mode renders it undetectable by other Macs.

8- Resetting the power management did not help. However, when I reset the P-RAM, I did get the Apple logo upon reboot instead of the spinning circle, only for it to disappear after a few seconds; the Mac then restarted, Apple logo again, restart, Apple logo, endless cycle.

So even if I wanted to format/reinstall MacOS on this MacBook Air, I am at a loss how to do so since I cannot even boot from an external source (USB or internet). Again, Windows *does* work. It is as if the Mac's firmware has been zapped to an extent that it doesn't recognize *anything* Mac. I would appreciate any clues. Thanks.