The Petrine Cross

I was recently havin a conversation with someone and it came up that someone she knew, after leaving an ash Wednesday mass, appeared to have an upside down cross on her forehead. I failed to see how the catholic would be shocked or offended at this insinuation. Apparently, my look of confusion also confused the person telling me this. "It's a satanic symbol," she offered. I explained the biblical story of Simon Peter and how the upside down, or Petrine cross, was a symbol of supplication and unworthiness. For any interested who don't know, St. Peter wanted to be crucified upside down, as he felt he didn't deserve to be crucified in the same way as Jesus. Many theists will know this story, but a catholic above all will be very familiar with it. The pope is connected to the Petrine cross, so I'd imagine the catholic felt honored. The other half of my conversation insisted that it was purely a satanic symbol and that I, as an atheist, probably don't know the bible well enough to argue against it and, moreover, shouldn't quote the bible in arguments/debates/discussions. I would like to put it to this fine collection of nonbelievers for a response. What do you think?