I agree that her boyfriend convincing her to go to Mass with him is a sign that he at least believed that some form of non-rational persuasion would work (since Mass isn't really about making rational arguments for Catholicism). Still, it's not obvious to me that this was the cause of her conversion. I'm guessing a much bigger factor was what she mentions in the next sentence: the deal they had where they would exchange books arguing for their respective positions.

I think you're underestimating the intellectual strength of Catholic theology, especially of the contemporary Thomist variety. It's miles ahead of any other religious apologetics I've encountered. I've read some of it, and while I'm not even remotely convinced, I can see how it could be extremely convincing to very intelligent people. In fact, I suspect I would have been much more susceptible to conversion if I had read some of this stuff earlier in life, before I read a bunch of philosophy (pragmatism, actually, hence my username) that basically inocculated me against it.