Good luck finding an attorney willing to take that on without being paid up front. This is not job discrimination or housing. It is a religious organization and it was a free event. Even if you successfully sue and win, damages are virtually nil.
The Catholic church has deep pockets, especially if the Diosese was aware of the discriminatory practices and condoned it...and there are civil rights attorneys who will write a letter/file a suit pro bono, others might do it for the publicity. Free event or no, damage award or no, it is the prinicple of the matter. A call and follow-up letter to the priest and the Diosese is in order.
I still recommend the woman gather a bunch of girlfriends to crash the event...if it is a free public event, then she has every right to attend.