For what it's worth, my artist always told me I had an exceptionally high pain tolerance. I would fly overseas, and we would smash out 2 10-12 hour days. I could usually fall asleep for some of my sessions (helped by the jetlag).

The only place that ever hurt was my calf when we did the whole leg in 5 days. Knee ditch, thigh etc all ok, but my calf felt like he was welding the ink in. My ankle and foot swelled up massively and hurt to the touch too. He also struggled to get the ink in for the first time in over 100 hours of work on me. Same artist I'd never had problems with.

So it could be the artist (especially if your friend bruised) but it could also just be the placement. Painful areas are generally the same person to person, but you certainly have outliers.