A friend of mine who travels internationally mentioned that his phone changes
the source addresses of received text messages based on the country he's
receiving them from, and that this makes it difficult to keep track of
conversations.

This project provides a script to work around this.
It takes an xml-formatted dump of the text messages database (from android's text messaging app, I presume)
normalizes all of the addresses, and dumps the result to a new xml representation
of the DB, which can then by reloaded into the phone manually.

Totally untested.