The same way he saves any type of file. Saving doesn't depend on type.
If the question is "how can he open it", that's different. He needs an app that can read text (or read and interpret xml) and that the system recognizes as having that ability. (And if he's going to read it in a text editor, he's probably going to have to rename it from .xml to .txt, since we can't just right-click and "send to" or "open with".)