I don't know anything about tattoos, but maybe it's a good idea to send him some practice skin, this is <$12 on Amazon https://www.amazon.com/Sheets-Tattoo-Practice-XLarge-Count/dp/B001CJSKUK
I hope he's doing alright otherwise
It's a brand. The skin is burnt and then agitated to produce a raised shape, similar to scarification.
This version of the sketch shows that the extra appendages appear to be faded as though they are an after image or motion blur of the woman throwing herself into the skeleton. The left leg was standing, then she wrapped it around him (yes, I just assumed the skeletons gender). The extra fingers are a motion blur type of effect. Same goes for the whole right arm, it was at her side and now is around the neck.
I'm currently on my second pair. They last forever, are surprisingly comfortable, and even have a bit of arch support. People think the price tag of $50 is crazy, but I say they're worth every penny. My first pair lasted 3 summers of heavy use and my current pair are in their second summer.