Jay, I can certainly empathize with your concern for your friend. However, as a practicing Catholic I can assure you that your friend will not be surrounded by people trying to convince her that she needs to "repent" of her sexuality. There's less that I can say about dark side epistemology (since you would probably consider me to be an adherent of it!) but I can assure you that Leah is not going to have piles of nonsensical doctrine shoved down her throat. She will be introduced to many ideas, but ultimately she herself will decide what to accept and what to reject (and I highly doubt that she will accept absolutely everything that the Church teaches - many Catholics don't).

I must confess that, as an outsider to (but occasional reader of) Less Wrong, I find certain statements and arguments on this site to be just as totalizing and dogmatic as the most dangerous religious fundamentalism. There's also a fair amount that I find personally offensive to my value system. However, whenever I find myself going into a nasty tirade against LW, my atheist rationalist friend (who introduced me to this site in the first place) urges me to remember that not all Less Wrongers are the same. Opinions about things like the Singularity vary greatly, as do values. And, there are even some theists on this site.

I can tell you that the same is true about Catholicism. It's a very large organization with many people who interpret their religion in many different ways. Yes, there are many things wrong with the Church as an institution, but people know this and some are trying to reform these flaws (indeed, if Leah does convert, she will be a great one to do this). As for the epistemological side...I don't think that Leah is going to reject scientific truths, if that is what worries you. She might just come to view them in a somewhat different way.

Having read Leah's blog for a while, I know that she will respond very well to any challenge/debate you put forward. However, I would advise against trying to thwart her conversion. Ultimately, she has to make her own decisions. As others have said, listen to her and try your to understand the reasons for her conversion. Also, if you haven't already, get to know a few Catholics. We could be wrong, but that doesn't make us bad people.