The green card itself should be all that is needed to enter the US as long as the holder has been abroad less than one year. If she has been out more than a year, she should have applied for a reentry permit before departing the United States. You can Google "US reentry permit" for more info.
Sorry, I didn't express myself clearly - I was just noting that the situation you described was an out-and-out scam, while the one mentioned in the OP was just a case of accepting a bad exchange rate.
You can absolutely get a Non-O from a Thai embassy or consulate overseas, though not all offices will issue one for every purpose. You may be thinking of the fact that some Thai embassies won't issue a Non-O for retirement, but that's due to a local policy, not a universal MFA rule.