Thing about trying to rationalize is that with a disorder like that rationality does not work. I used to go through that and in my head I totally knew that there was nothing to be afraid of, but deep down inside the anxiety got to me and I couldn't. It's like the prayer rituals I used to have with my OCD - I knew that if I got my prayers slightly wrong it doesn't meant the person I forgot to pray for was going to die - but I'd obsess anyway thinking "well, what if I'm wrong?". Your friend needs to want to get past this in order to. Just be there for your friend and be encouraging. Truth be told, the rationality stuff rarely works and for some makes them feel bad because they know they should "know better" but they just can't do it.