szb1: It really has a lot to do with the individual person – are they more emotional or more intellectual? What are their reasons for being anti-religion? You may have to listen to them first and see what they say and where the conversation goes, first empathize with them and then steer the conversation accordingly.
I would probably talk about the purpose in life. Point out to them that there HAS to be a purpose in life. Maybe ask them what they think life is about? Point out that it ends one day, and then what?
But the specific approach may have to take a more emotional bent if the person is a more emotional type. If they are more logical and intellectual, you can focus more on the intellectual reasons why it makes sense to be Frum, and if she is more emotional, you may have to talk more about meaning and fulfillment.
Aish HaTorah put out a video years ago about doing kiruv. I never saw it, but it sounds like it could be helpful. I think it was called Inspired or something like that.