Check in documents for a 15year old passenger

I would contact the Airline company and ask them, I wouldn't take someones word for it and find out later they were wrong.

For international travel, a passport is required, even for children under 18.  For travel within the US with an adult, no travel document is required except the boarding pass.  However a picture ID may be helpful.  Traveling unaccompanied may require special arrangements; almost always for children under 12, but it depends on the airline whether 15 is old enough, and whether there are connections.