Let's say he grows a beard that covers the lower half of his face but on his passport he doesn't have a beard or maybe it's the other way around or let's say maybe he got involved in an accident that made his face look different or under went plastic surgery does he need to get a new passport done?

A beard or not, long hair or short hair is of no consequence. Immigration and police officers are used to that, it happens all the time

Plastic surgery can become an issue as that can change your characteristics, in that case it might be advisable to get a new passport.

That is precisely why young children are sometimes asked to get a new passport as they age..... quite often a 3 year old does not look like a 30 year old.

Americans have ten year passports. A lot can change in 10 years. Been through 4 of those and never had a problem.

It doesn't matter in Thailand, we all look the same to them.