My model of her was obviously mistaken before today, but I find it hard to believe that someone could go from atheist advocate to committed, full-strength Catholic and yet stay fundamentally the same person.

On the contrary, I'd say that if your friend was always committed to philosophical truth in matters of religion, then her conversion to a fully committed Catholic is more "staying fundamentally the same person" than becoming a cultural, belief-in-belief style of Catholic would be. She has just reevaluated her assessment of some very abstract philosophical arguments about metaethics, metaphysics, reliability of testimony of miracles, etc, and followed her new assessment to what she saw as its logical conclusion. This need not imply any direct change in her basic personality, whereas changing from a committed truth-seeker to a "pick and choose what feels nice and go on my way" would imply more of one, I think.