With the ever changing travel policies/restrictions, best bet is to just call the consular office. I just looked up https://ph.usembassy.gov/covid-19-information/ and it was updated last week. It looks like if a US citizen travels with a Filipino citizen that is either a parent, child, or spouse, they can be allowed to enter the PI, as long as they have a visa as supposed to without a visa like what we did last December, I traveled with my wife (Filipino citizen). Also, by calling them, he'd have a realtime answers on his inquiries, especially in an emergency situation like he's in. Since I'm not time constricted, I personally applied for a dual citizenship, so we really don't have to scramble for a visa if we have to come back to the PI for i.e. emergencies.
I doubt the embassy will answer this so I'll give it a go. All your friend needs is his US passport, unless he wants to stay more than 21 days....
Thanks! Your answer is awaiting moderation.