I have a friend in the UK who wants to marry his long term girlfriend when he returns to Thailand but is unsure what he needs to get & from where to prove that he is single & free to marry.

Any information appreciated.

You can probably find the information on the UK Embassy Bangkok website.

Don't need anything to prove your single. Only need proof of divorce. You just make an affidavit.

Never saw the logic, as if you're going to lie about being divorced, you may as well lie about never being married. It's still bigamy.

See Royal Thai Embassy's General Guidance for British Nationals wishing to marry in Thailand,

British Embassy's Marriage Guidance and

Department of Local Administration's Marriage Registration.