[Executive summary: solve the underlying causes of your problem by becoming Pope]

I think it's a mistake to focus too much on the case of one particular convert to Catholicism simply because you know her personally. To do that is to fall prey to the availability heuristic.

The root cause of your problem with your friend is that the Catholic Church exists as a powerful and influential organisation which continues to promote its weird dogma, polluting Leah's mind along with the minds of millions of others. Before investing time and effort trying to flip her back to the side of reason, you should evaluate the costs and benefits of destroying the Church as an effective entity. I will now outline a method by which you and around 20 like-minded friends could do just that.

The Catholic Church is based in a tiny pseudo-state called Vatican City State. It has no permanent population and no true army, the Swiss Guard being more of a ceremonial bodyguard force (although they do have modern firearms as well as the cool-looking pikes).

What I propose is that you wait until the current Pope dies (not long now!) and a conclave has been assembled, then rush Vatican City in an infantry-style terrorist assault. There are 150 or so of the Swiss Guard but you could divide their forces by having some of you occupy a building, display simulated explosives and make fake demands. Your true targets are the cardinals who are there to elect a new Pope.

Once you capture the cardinals, simply force them at gunpoint to elect you Pope. In the event that you're not already a Bishop and therefore not an eligible candidate for the Papacy, simply mount a privilege escalation attack, whereby you force them to elect you to successively higher offices until you become a valid Pope. I anticipate that this process will be completed before the Italian state can mount an effective special forces operation to kill you.

Now you are Pope, you are the sovereign of the Vatican City State. You can pardon your co-conspirators, and appoint them as ambassadors so they have diplomatic immunity outside VSS. You can then use your papal infallibility to remove all the problematic doctrines of the Church (homophobia, opposition to birth control/abortion, etc.) and bring all the child rapists it has shielded and enabled to justice. Or you could change all Catholic doctrines to those of Pastafarianism. Either way, the appeal of Catholicism to your friend would be destroyed as its so-called timeless moral insights are revealed as human constructs. One or (preferably) more "True" Catholic churches will arise to challenge your claim to the Papacy, causing decades of damaging, hilarious schisms, during which you should make sure to declare several Antipopes.

I suggest you treat this post as if it's a joke, and then seek military training as soon as possible.