there is only one embassy in the USA and like all other embassies in the USA it’s in Washington DC. The others are consulates. This is visa 101 and taught in elementary school social studies
if he already has a non O extension then he must have a bank account. So another option is to enter visa exempt and get the non-O issued in Thailand for ฿2000. Then extend off that.
Another thing I would suggest is to do this by yourself with minimal help from your Thai girlfriend.  Going into immigration speaking half Thai and half English stuff can get really confusing. I’ve seen it a lot. Stick to English and do it all by yourself and you’ll be better off in the long run. Thai people know very little about Thai visas just the same as an American wouldn’t know much about US visas since they don’t need one.
If you already have a bank account just transfer the money over and get the Non O issued in Thailand. I got my Non O in Thailand based on retirement it’s 2000 baht. The only reason to apply for a non-o in Laos is if you don’t have a bank account already.