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.

But as to the "I don't see what the big deal is" point (which a number of people have raised), I do have some concern, because I know how seriously my friend takes integrated belief systems. Some have suggested that this may be a "belief in belief" issue, but that's definitely not the case. She's very much committed to there being a truth of the matter here, and my guess is that if she really did commit to Catholicism, she would be more like a "full-strength Catholic," not a "pick and choose what feels nice and go on my way" Catholic.

This seems like a more substantial objection to her conversion. If she becomes a full-strength Catholic, do you see her arguing against abortion, homosexuality / same-sex marriage, responsible birth control, using condoms in Africa, etc. because she'll see those acts as being against her ethics, since now her ethics are going to be that of the Catholic church? Those are actually harmful beliefs (and actually cause people to die in the case of anti-abortion legislation and condom use in Africa), and should be thwarted as strongly as possible. But then again, you might be able to just argue against those points and not so much her Catholicism.