Wtf does god have to do with the 4th of July???
I can probably draw a connection between anything and anything else. So here goes for this:
We celebrate the 4th of July because of the adoption by Congress of the Declaration of Independence, which states that our right to revolt came from the "Laws of Nature and of Nature's
God," and that "all men are created equal, that they are endowed by their
Creator with certain unalienable Rights.”
The Declaration has its roots in the laws of God, even though Jefferson had his own quirkly beliefs in that regard. So your tattooed friend understands the meaning of atheism much better than most atheists. (Probably not; but in his ignorance he just happened to actually be making a good point.)
Most atheists don’t have a problem with celebrations that have their basis in Ye Old Time religion, but there are some who are extreme and want to make a point. They not only don’t believe, which is a personal thing, but also hate religion and anything to do with it, and would like to see it abolished. Your friend seems to be one of those and it is a legitimate position.