so the obvious question might be, was your friend any more healthy than you at the time? less acidic, would you say? because it does seem that the more finely-tuned the body, the more likely it would be to reject the tattoo, initially.
Or-- the other possibility could be, that your friend was closer to his upper limit re: poisons... hence, the rejection.
As yours have not faded, well...but there's nothing to stop you purifying yourself
as a wonderful experiment. :-)