hi,
i think he needs to get a status paper form his embassy. To do that, her future wife must get her birth certificate and her status papers which can be obtained from her hometown amphue. Some amphues also give the documents but some asks to go and get from her hometown. After she got them, you guys need to translate them into english and certified from the minister of foreign affairs. Then take them to your embassy. After you submit those papers of hers, you can ask for your status paper which says you are not married in your country or here in thailand or anywhere else and you can marry with her. Now with all these documents again you need to translate them into thai and certified from the minister of foreign affairs. Now you can take these papers and go to any amphue you want and get marry. It will take probably 15min.
The steps i told you are for almost all countries dont know if your friend embassy want or not. probably want.