My advice is as a proffessional tattoo artist- DO NOT let your friend get a tattoo at 15! That is ENTIRELY TO YOUNG!
I can give you a litany of reasons why it's unethical & unsafe to get a tattoo at that age, but you probably wouldn't listen, or care. I could tell you that no 15 year-old knows what they will want on their body for the rest of their life.I could explain to you that no tattoo artist worth a damn would consent to tattoo a 15 year-old and any artist who WOULD is going to be terrible and give your friend a bad job; but HE/SHE probably wouldn't care. I could tell you that it's ILLEGAL in most places to get a tattoo before you're 18; but you'll probably tell me you live in one of the few places that are still behind the times and allow any age tattooing.
Here's a reason you might listen to: at 15 there's a possibility that your could have a growth spurt (whether you think it likely or not) and if that happens, the tattoo will get COMPLETELY messed up!
Look, this is what I do for a living- and I'm not in the habit of turning down money by discouraging people from getting tattooed- but this is one time when saying "NO" is THE RIGHT THING TO DO! If ytour friend really wants a tattoo and this is the design that he/she really wants, it'll still be around in 3 more years when it IS APPROPRIATE to start getting tattooed. If this person is really your friend, you'll convince him/her to wait and to go to a top-notch professional when they turn 18.
If you happen to live in Texas- and you get him/her to wait- I'll do the specific tattoo you're talking about for free.
Hope you make the right decision- actually, I hope your friend makes the right decision!