It is very difficult to get a UK entry visa for marriage from SE Asian countries (too many young eejits trying to marry bar girls, sad but true). A lot depends on his ability to financially support them, and convincing the officer that it is true and lasting love. To be honest, if it was a casual relationship at the time and he hasn't seen them since, his chances are next to zero. If he can show he has travelled to see them regularly, and he has been supporting them, he might stand a chance on appeal.
Alternatively, and if he is that desperate, he can get married in Laos under Laos law, and then registered the marriage with the British Embassy. It would not make approval for her automatic, but might help build a case.