Kat Von D has a waterproof foundation & other products for tattoo covering or enhancing. That is her thing as she has tats. The foundation has a lot of good reviews. I was going to get it for a sun spot on my face that has darkened a lot.
I still haven't tried it, as the Ulta sales person lured me to try Lancome's Teint Idole Ultra Liquid 24H Longwear foundation instead. It really is long-wearing & waterproof and gives full coverage. 2 layers does the trick.
Have DD watch some YouTube videos on how to cover the tattoo or to apply a "full coverage" foundation. That is the coverage she will need. It's best to apply it with a makeup sponge and DAB it on, rather than rubbing it on. Let dry. Dab a second layer. It may need setting with a transparent powder. Liquid foundations are liquidy and may need that extra powder to grab onto the skin & set in place.
Since it's on her shoulder & not her face, just get the cheapest compact of transparent powder from the drugstore or dollar store. Once the foundation is dry, just pat on some powder with the powder puff. If she is finding she is sweating and the the foundation is wearing off, a couple pats of the powder may be what keeps it in place, rather than having to apply more foundation.
BOTH of these foundations are very expensive. If DD goes to Sephora or Ulta, she can ask the salesperson to help her apply it and then ask for a free sample. They'll give her a tiny amount for her to try on at home. It's usually enough for 3-4 tries.