if the law for Covid extension is different (or if he is guessing). Because if you are on Tourist visa or visa exempt entry, you can extend while on overstay. You will get stamped 30 days which include the overstay days. So if you are on 10 days overstay, you get 20 more days for 1.9k baht + 5k baht overstay fine = 6.9k baht.
You can self-isolate if you catch Covid, you do not need to tell officials nor will get into trouble. The only trouble is the overstay fine which she cannot avoid.
OP husband tested positive 10 days ish ago, it's way too late to contact an hospital to get a letter to avoid overstay fine. Also, the hospital fee can easily rack up to 6 digits (100k baht+ for hospital stay). This is a common scam that I've warned everyone about (getting stuck in hospital when testing positive for Covid). Finally, she is from Myanmar, a country currently in a civil war, her embassy is worthless. She has a better chance at seeking asylum then getting help from her embassy.
I know that if you are on Visa exempt entry or Tourist visa, you can extend 30 more days for 1.9k baht, even if you were on overstay. I did it a few time myself. If it doesn't apply to Covid extension, well, you are looking at a 23k baht penalty upon leaving on April 19.
I've extended my visa 30 more days while on overstay before but it was never the Covid extension. You know people who got denied the extension due to overstay?
Thailand Pass is NOT a visa... it's a extra layer of requirement due to Covid. You came into Thailand without a visa so because you are from the USA you get the 30 days visa exempt entry stamp.