If he comes in on visa exemption, his SETV would still be valid for 3 months, though. He could leave and re-enter with it (before the 3 months are over).
Maybe possible to get COVID extensions. Like you said, if it isn't possible, he just leaves like he was planning to do anyway (and maybe even gets 7 days). Worth a try. And worth not getting stuck in Cambodia due to Omicron. 😄
10MBPS isn't slow. The issue is upload speed for video calls. Usually companies will advertise something like 50MBPS download speeds, but only 1 have MBPS upload. You need to check both download and upload speeds.
Depending on how many days you overstay, you can be blacklisted from Thailand. Since he would overstay 25 days, he wouldn't be officially blacklisted, but I believe his overstay would appear in the system every time he would try to enter the country by air—which would drastically reduce his chances of being let into the country. Basically, making him blacklisted.