As far as I can see his wife would just need the normal Visit Visa, are they going to be in the UK less than 6 months ? Has the Lady had any issues with Foreign immigration IE refused entry to another country ? If not full steam ahead, just guessing they will be traveling out / back together ? Reason to return: they reside in TH together and they / she owns a business etc.
Good luck to them, be nice if they planned to be there, the day of Scotland's Summer.