If GarminDevice.xml is corrupt you can try to edit it and try then Webupdater, if you have a good .xml file from the same model use it changing unit ID. But must see why it has been corrupted, your friend doesnt tell you the whole truth about what happened to this unit
Probably he flashed something really wrong to brick it like that. 37xx is hard to brick completely, usually kunix's cure solve all problems.
In original GarminDevice.xml check HIWD if it is the same than yours.