I think she would struggle to get regular commissions for tattoo designs unless she was doing them herself. Most decent tattooers won't copy a design without redrawing/changing it, if at all. I'd also add that I don't think some of the very fine lines in her work would translate well to skin.
Edit; re-read that and it came across quite negative, I do like the designs! If she wants to get into tattoos she should look at getting an apprenticship, if she doesn't actually want to tattoo its probably a bit of a non starter.