You do not get arrested at immigration while trying to extend your expired visa. This is not their job.
The major way that people get caught is thought check-point, which is rare. Cops asked for my passport once in the last five years, and it was while taking the bus to Mukdahan border pre-covid. They are more worry about people that look like Cambodian or Myanmar people.
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?