Work with her to find a very reputable place and amazing artist to get it done. It is permanent as you know. Visit several places and talk with people.
Start researching her options and artwork and designs. How much is it going to cost to get it done right? You don't want just any john doe with a tattoo machine to ink you forever with a crooked, or badly drawn, or poorly designed piece of "art".
You need to find the best most reputable and CLEANEST place to have it done.
I was researching ear piercing for my daughter she was 6 at the time. And was going to take her to a local piercing and tattoo shop. Why, because their rooms looked like surgical suites and the health department gave them highest scores. (We ended up doing it at the pediatricians and I still kinda regret it because one is crooked.)
I think in the process of researching the whole thing and having time to visit with her you guys can really talk about the whole thing. Why you regret your tattoo...if she should wait a bit longer to get hers...where is she going to get her's placed...will she still like it at 25, 30, 40, 50...can she afford to pay for quality work...
Neither my husband or I have a tattoo (he was military and has his all planned...then when he had the money could never find time to go get it done...or when he went with friends didn't have the money...so it never happened. Now he is so thankful he didn't get it!!!)
We joke that now we are counter culture and "rebels" because we don't have ink. Because everyone we know has at least one...usually three or four.
I don't have one because as an artist myself, I could never put one design on my body forever.
Then again I have a friend who just got back from flying to Warsaw to catch up with a very famous tattoo artist to get his art done. This tattoo artist travels the world, people pay him large amount of money for his art. It is one of the most beautiful pieces I have ever seen but the cost to fly and the tattoo must have been very expensive...but it is forever.
Anyways...to visit places and talk, talk, talk...then if she does it, you both have you bases covered. Big hugs!!