First NOS is ok to have on your car. The illegal part comes from the time you hit the button (Exhibition of acceleration, racing, reckless driving, etc etc)
As a rule of thumb, only things that can be seen will be enforced. Tailpipe length, red reflector, windshield wiper, etc.
The NOS may become a inspection issue, however most of us only have a safety inspection so that would be a mute point anyway.
No cop (I know as I "is" one
) is going to pull out the 9/16 to remove your NOS injectors. Your friend is telling you another "woke up with my kidney gone" story.
Of course I do not include the wonderful people of the Texas Highway Patrol (otherwise known as the Texas Ticket Writer 2000!!!) as they seem to find anything to write you.
If you need to sleep one night, Look up section 545 in the Transportation Code, subchapter 1, Miscellaneous Rules, very interesting reading.
Todd