If necessary book a ticket to another country (bus or train may be acceptable) or contact the embassy and ask them. They will tell you what they will accept.
Robert Lagas I understood it to mean that she entered Thailand with her current passport but since it was relatively new they wanted to see her old passport too.
It's common practice to carry both the old and new passports for a while after a changeover.
I think they suspect that she has a substantial history staying in Thailand but they cannot see it at the embassy without looking at her old passport.