As usual Mono has a wonderful suggestion. Not the route that I was going to go, but upon reading that may be the better option.
I'm a fan of communication. You said that you've all talked about what can be done to help and you can think of nothing. What is it about her having a sexual relationship with someone that she loves deeply that shuts you down? That's the question you need to ask yourself, when you're getting ready to go to bed and you're going over the day. You know, that point when you're most honest with yourself.
Is there a feeling of inadequacy of some sort? I'm not implying that, merely trying to dissect the issue.
The cup is always half full in my world, so things will work out for you.
Live life to the fullest 'cause you never know if you're gonna wake up tomorrow!