My 21 Year Old Nephew Just Got Tattoos Onhis Arm. How Dangerous Is This And Are There Any Side Effects?

If he got a tattoo by a professional then there is no discernible risk as they sterilise their equipment . And he looks after the healing as instructed.
Tattoos have been around for thousands of years & in some very primitive ways of applying them too so by the ancient standards today's methods are safe as walking around your own house.
Being inked is becoming a modern thing to do some of us have buy prescription drugs on line many tattoos and have had them for decades before it became the in thing.
I have never regretted mine as I chose carefully other than some peopls narrow views still there is no long term side effects