Can my friend with a UK passport face issues re-entering Thailand after obtaining a tourist visa from another country?

Yes he could have problems.

I'm assuming the following for your friend:

1. Three visa exempts. One by air when he arrived, and two by land, all extended (180 days)

2. A tourist visa, also extended (90 days, or 270 days).

In my opinion:

1. Your friend needs to find a valid visa to stay in Thailand. S/he isn't acting like a tourist, so have a plan B.

2. If he didn't have a tourist visa from Vientiane, Laos yet, try getting one from there. They are getting strict about denying visas.

3. He can try getting a tourist visa from Savannakhet, Laos, but they may issue him a transit visa (assuming he's not denied).

4. He can try getting a tourist visa from Vietnam - I heard mixed reviews about it.

5. If he only wants to stay here for another 60 days and then leave, he can try re-entering on a visa exempt by air.

Of course, it would be helpful to know what part of Thailand your friend is in, and what countries s/he got her visa from.

So 9 months in Thailand on a tourist visa. At sometime immigration will start asking questions.

4 entries? Are they with a multiple entry visa, or different visa or without visa, did he use the multiple re-entry permit? How long did he stay, did he applied for Extensions of Stay? Does he actually live and work inside Thailand? What is the purpose of stay, does he has funds to pay for such long stays, does he has proof of onward travel and does he has proof of accommodation. Just a few things that the Immigration Officer ask and thinks about in the few minutes he got and than makes the decision that your friend can enter or not.

What are "4 entries"? Are these visa, visa exampts? And if they are visas, what type of visas, and from where?

He could indeed. BUT why doesn't he go to a thai consulate in say Vientiane or Savannakhet and then come in by a LAND crossing instead?

No one is hassled much at the Nong Khai or the Mukdahan borders entering on a tourist visa