I've done it, through the Content Screen, since day one and I have not had a single issue. And I continue to remove un-needed things after every Update. You'll be just fine if you do this.
Maybe because there were larger issues to fix besides a few lights out of place or a few signs missing. Oh and BTW, MSFS has been out for only 5 months.
1. Your friend is not correct.
2. Move the files back or uninstall MSFS and reinstall it.
3. You might have to manually set up the Attack3 in MSFS to have it work.