A friend of mine...

[Jason Jarred]

It's a problem alright, I can't understand how people can be together for so long, have a family etc, and still not have a combined view on religion. From my experience the more time I spend with people and discuss important issues, the more our views tend to merge as we persuade each other into a common ground. Maybe they don't communicate enough? Maybe the issue is too sensitive or awkward for them to discuss?

As for your role, I wouldn't give any advice except to actively listen and empathise with him. I've always found that when people are distressed or need to talk about something they don't really want advice, they just want somebody to listen and show that they're listening, and understand what they're going through. These are some rules that work for me:

1) Don't give advice
2) Don't change the subject
3) Don't analyse what they're saying
4) Do reflect back what you've heard to make sure you understand the emotion *and* the logic
5) Do genuinely *listen* and try to understand the emotion behind your friends words

I don't see it so much as a religious issue for you, more of a relational issue between you and your friend. If you had to offer any advice, I would gently suggest for them to see a counsellor. I went to a counsellor with my partner two years ago when our relationship was in dire shape, and things have never been better (we celebrated 7 years together in March). It all comes down to how well we communicate our emotions, our needs, our thoughts.

I'm amazed I could find the link after all these years, but I seriously recommend this resource to anybody interested. This is a link directly to the "Listening and Empathy Responding" article, I recommend at the very least the entirety of Chapter 13. Here's an excerpt which shows the benefits of empathy responding to friends or loved ones (which I personally can verify):

• It shows you care and that you understood the other person. Thus, people will enjoy talking to you and will open up more.
  
• If you have misunderstood, the talker can immediately correct
  your impressions. You learn more about people.
  
• It usually directs the conversation towards important emotional
  topics.
  
• It lets the talker know that you (the listener) accept him/her
  and will welcome more intimate, personal topics. It invites
  him/her to tell his/her story and vent his/her feelings.
  
• Since it is safe to talk about "deep" subjects, the talker can
  express feelings and self-explore, carefully considering all
  his/her deep-seated emotions, the reasons for those feelings
  and his/her options. Thus, it is therapeutic.
  
• It reduces our irritation with others because we understand. To
  understand is to forgive.
  
• It may even reduce our prejudice or negative assumptions
  about others because we realize we now have a means of
  finding out what another person is really like. Furthermore, we
  discover everyone is "understandable."

This has been a huge help for me over the years and I can assure you that it provides some practical and real world advice that you can actually put to use and see that it works, it's not all just theory and pontification!
I hope that is useful to you.