People who want to break up with other people don't "need" anything. They just do it.
She wants to complain to you. She wants you on her side. She wants you to be available to drop everything and show up for her to double team the guy or the gurl or someone should she decide to confront them. She wants you to be equally pissed and equally mad and ready to back her up at a moments notice, or open you door and comfort her while she cries/complains/whines. She wants to brainstorm revenge fantasies and manipulatory ideas with you. And when she jumps back into his arms/lap/bed she wants you to go home, put away all of your anger and pretend like nothing ever happened. That is until the next time she wants a shoulder to cry on.

She's using you to vent so that she can stay in the relationship. Every time you're there to listen she dumps everything on you, walks away fresh and clear-headed, then goes back to him ready to deal more. Sometimes you just have to say it's none of my business.

She doesn't need proof to leave him. She needs you to believe she needs proof so you'll keep listening. You've been manipulated into playing a part in their relationship, you get the trash dumped on you. They're both manipulating each other and outsiders to keep themselves entangled. Sounds like they're perfectly matched.