It might not even work! Tattoo's are just pictures and to do a spell you might need OTHER things then that picture. If it is a talisman then maybe it could work out, but remember... Over the years the skin grows and spreads out, same with the tattoo. So by the years it could look like a silly face or a blab, which makes it useless.
If it is a spell that needs stuff like oils or herbs then it could work if he adds it on, by why get a tattoo about it when he can just get some chalk and do the picture or words he needs onto their? And if it is just words then why not remember them besides going as far to get a tattoo? Unless he had a very good reason to do so, he will make the dumbest mistake in his life.
Anyone that says otherwise don't know what their talking about and just trying to show their face around the site, but there are very better ways in to do a spell then to tattoo it on your arms, legs, or chest...