Paintings, illustration and drawings are great for reference and you can reproduce the motif in them just fine. But it gets tricky if you want reproduce the look and feel of other mediums as tattoos. A soft, subtle watercolor or a sketchy charcoal drawing make bad tattoos.
All mediums have a natural look to them. Watercolor, oil painting, charcoal and tattooing. Now, I can force a watercolor painting to look like an oil one, but it wouldn't be in the best interest of either mediums. Same with tattooing. You can reproduce your friends painting but make it look like a tattoo.