on the other hand, as a person with the means to make such drastic and permanent changes to a person, the tattoo artists i know consider themselves to have an ethical code, like surgeons or doctors. you give a person a tattoo and they regret it and bad mouth you, you might to lose out on customers.
also, i think your friends tattoo is cool, and i don' think it would fall into the same category as the sort of "random-meme-XD" as the sorts of things he refuses. having a zombie and 'graaaaaaaains...' sounds like it was planned, well thought out, has a meaning behind it.
while i was getting my shoulder done (on a break for tea and biscuits) a girl showed up with a picture on her phone and wanted a tattoo saying 'love' on her wrist. she'd just found a picture of someone elses tat online and wanted the same thing for no thought out reason. he pretty much flat out refused her, telling her to go and have a proper think about it. i think if you have the power to change someone's body, then you are partly responsible for those changes. also, tattoo artists are artists... no artist would ever be forced to undertake a commission or a musician play a gig they thought was horrible/tacky/juvenile unless they needed the money, in which case it is a personal decision.