A friend of mine has got himself into overstay of 1 year, is it true that if he gets a ticket to KL flys out paying the 20k fee on the way out then getting a new B visa in KL will he be allowed back in after that?

Yes. Just make sure he doesn't get caught on the way to the airport. But once at the airport check in for the flight and go through immirgation as soon as possible. he will be send to an office, where they will fine him and mark his passport with overstay. Then he can leave and re-enter Thailand.

Most likely if he is not wanted for some other violation.

One year overstay on a non immigrant B visa? Was it a multi or single?

So how has your friend got a valid work permit, unless he or she owns a company.

ok thankyou for the quick response.