There's several ways this could go. Personally, I've never heard anyone say that tat's are a distraction and shouldn't be a reason to sit any one down. I have some myself, when I wear short sleeve shirts. They are visible NEVER has anyone said anything about them. If she wasn't informed of a particular dress code for singing and showed up wearing what she's wearing what's the big deal? You stated your church has been on a 'summer dress down' implying that everyone has been dressing casual for however long, then she shows up casual? okkkkay??? You know why the majority of church members steer away from the church? Because they've felt uncomfortable at some point (i.e. backstabbers, people talking down to them, being hurt by fellow members and in this case getting questioned about their attire).
You have a problem with the fact that she has a tattoo, you said so yourself. If she didn't have a tattoo would you still be tripping? I'm glad I'm not a part of a judgmental congregation and I thank God for it.....