Writing Contemporary Womens Novels

For those of you who read or write comtemporary womens novels, I have a question.

At some point in my story, my protagonist has to discover her friend is having an affair with my girl's husband. My problem is where abouts in the book this should happen.

When I watch a TV drama with this kind of story line, I quite like knowing what's going on before the heroine finds out. But in a book written in the third person, this is harder to do as I really only have the protagonist's POV.

As the antagonist is really more the emotional situation - a marriage breakup in the first chapter - can I leave this 'discovery' to later? And - how could I let the reader in on it without being omniscient? A few lines of prologue before the relevant chapter?

I should write it and see? I thought you might say that. I guess I just wanted a quick solution.

I would still be most grateful for your thoughts or ideas.