I was worried about TG e-ticket credit card policy until Retired FF's last post. I understand that you purchased the ticket with one of the passenger's credit card and they should be able to check-in and board the plane without problem. You may want to contact TG reservation office in bangkok which is available 24 hours at this number 66 2 356 1111. I am not sure if they can change the passenger's name, but they may be able to add note for the passenger record or give you further instruction.

I hope this helps.