What issues could my friend face due to a 13-day overstay in Thailand?

Good luck to the O/P's friend.

I think they can figure out what to do

His work permit has nothing to do with immigration, that's the ministry of employment,

His extensions is his main concern,

500 Bhat a day fine for overstay,

There is also no safe and guaranteed way to avoid a fine,

It's up to immigration at any border as to whether he gets back in or not.

Penang is not the place to go as they are the only Thai Consulate that does not issue a new visa to people with overstay stamps.

Foreign wife and child. What kind of visa or Extension of Stay are they using, when they use an visa or Extension of Stay based on a foreigner working in Thailand, piggyback on his visa, their extensions of stay also became invalid and are in overstay.

First off a valid "work permit" doesn't let him STAY in thailand, it allows him to work IF he has the correct visa.

Secondly you don't renew a visa by going to the border OR going to a thai consulate.

How about stating what type of visa he's on now, and what type of visa he's thinking of getting in Penang?

As people pointed out, it's NOT the overstay that is inherently dangerous it's getting caught out on something else inside thailand while on overstay that runs you off the rails.

The way the overstay rules are written you don't get blacklisted for overstay when turning yourself in (which is what you do at the border or the airport) until you have an overstay OVER 90 days

BUT

It also states that if you're caught inside thailand on an overstay of even ONE single day you can be blacklisted for 5 years.

Also as people wisely pointed out FLYING out of the country is the least risky thing. Have him show up at the airport ready to pay the 6500 baht overstay fine, get the overstay stamp in his passport and fly to a nearby country to go to a thai consulate to get a visa.

That's 500bt for every day and better sort it out quick before it goes to a fine with a 5 year ban.

Taken from internet

So if working he is doing so illegally

He does not have a valid work permit, because he has no permission to stay.

Instead caught but the police in his care ,before the border ...he should fly from phuket to had yai.....then go by trains to Penang ...pay the overstay at the border...

If his passport is checked before reaching the border it's a major problem: detention and deportation.