This might have been stated before, and if it has, forgive me.
But as a mother of a 16 year old, I know the struggles that some parents go through in getting their children to attend Mass. The fact that this young lady was there is a wonderful thing. I believe with my whole heart that God can speak to even the most hardened and cold and lost of hearts, if only we give Him the chance.
Though this shirt was wrong to wear at Mass and maybe the priest should AFTER Mass point this out to her, in a very kind and loving way I might add, at least she was there and as such, the opportunity for her to hear the word of God and experience His love was made all the easier.
Take this as an opportunity to pray for her and all the young people who are trying to find their way. Ya never know, God may be using this young lady as a way of making You a Saint.