Those from the Philippines quality for visa exempt with 30 days on entry (possibly 45 now with the recent proposed extension). Visa on arrival is a different form of entry - yes, VOA is not currently available but that doesn't apply here.
As others have noted, she'll then have to do 14-days "home quarantine" (the official term used in English) upon traveling to Chiang Mai.
even before I had two year visa extensions and a work permit I was always looking out, well before COVID as to the best way to stay here long term legally for longer periods and I've been through multiple visas since 2013 :)
I don't want to be unfair - there may be a few who are legitimately stuck here but many of these people are not even trying at this point. In Chiang Mai, it's not that expensive to enroll with a Thai language school or even a self-defense class with the equivalent of a year-long ED visa (90 days + 3 x 90 day extensions) if people really wanted to stay without drama. For those over 50 I believe Non-Imm O's are available within Thailand now as well.
my bad - COVID extension or whatever they're calling it this week ;) Op here obviously is coming off a Non-Imm B so is in a different situation but what has me beat is with all the amnesties and now COVID extensions, anyone stuck here has had ample time to find alternative visas or leave the country. With some exceptions most claiming to be stuck here can leave the country now but they don't want to.
You might qualify for an extension of stay under the visa amnesty (note - this is not a visa) but there's zero way you can apply for a tourist visa within Thailand even with all the others things going on due to COVID-19. To get a tourist visa you have to apply outside the country.