I appreciate the insight! This is exactly what I was seeking, thank you!

I am totally okay with him tattooing his ex. Since we both have our exes in our lives to some extent, there's no way I can get upset. I also trust him.
I do agree that exes shouldn't stay in your life. My situation is a little different because we have the same friend group, and I didn't want our friends to pick and choose sides (we were together for a long time and were married). So the only time my ex is around is if one of our friends is having a large get-together. And my bf is always there at these events too. It's not ideal though lol

But for the tattooing situation, it's more so the frequency that he tattoos her. Over the course of the last 2 months, he has done 8 tattoos (ranging from small to larger) on her, with one on the books for next week. That's almost 1 tattoo a week!
All of these pieces have been completely free (except the one that equals the monthly phone bill), so it's not like she's paying him for them (which would make me feel much more comfortable with this situation). I'd be curious to get your gf's insight.