It's hard to separate out your childhood experiences here.
Is your friend a member of a socially conservative congregation, and expressing her true self for the first time? Is she seeking to fulfill something lacking? Was she just bored? Without this type of context, there's no real advice to give.
You have to follow your heart here. You have a good one.
She, like me, grew up LDS and we both resigned our memberships nearly 10 years ago. Her husband is still active LDS but they had worked through it. Oddly, her new love is also LDS.
She and I are about as different as we can be. She has always pushed boundaries while I am a rulekeeper. After leaving the church, she went through a mild rebellious period (getting tattoos, getting drunk, etc). I did not. I love her because she challenges my thinking, and we have always loved talking to each other.
She still loves her husband and doesn't want to leave him. She has tried to talk to him about this, but he has not wanted to have the discussion, understandably.
Most of her kids have met the woman, but so far seem to think this is a new, intense friendship. She is going to become a grandmother in a couple of weeks and realizes if her family figures out what is going on, she may not get to see her grandson much. Her children adore their father and would be devastated by this and would likely not be too forgiving.
She is risking her entire family, and she knows this. Yet she can't leave her. She wants to believe that somehow she can have this.
How I've handled it so far is to listen. I've told her that all I can see ahead for her is a lot of pain for everyone she loves. I'm not sugarcoating my feelings, but I'm also being gently reasonable. I've also reassured her that I will be in her corner, no matter what happens.
What I see is that her marriage has become a victim of time. Her husband doesn't cuddle with her except for when he wants sex. And she says sex with an old guy is a lot of work. He has not taken care of himself and is kind of a big, bearded, messy guy - but perhaps the kindest man I've ever know.
I suppose it is true - there probably isn't any advice anyone can give me. I'm just going to have to watch this soap opera play out. She knows this is a bad idea and is choosing it anyway. It's an incredibly selfish decision. And I can't stop it.