I would do it because I think she has a right to know...My view is that it's better they have all the information in order to make a good, informed choice...I am personally satisfied that the responsibility lies entirely with the cheating partner...
You think, you would, you personally are satisfied.
But also...
I would want to know, others wouldn't. None of us know what is in another person's mind
Your response to this is to to with what you decide, you think, you are satisfied with. But if you understand that you don't know what others want, and that some people wouldn't want you telling them, your personal view isn't enough to make such a massive unilateral decision for them just because YOU would be happy with doing it. It's actually quite a myopic and self serving view.
If you're actually involved in the situation, that's one thing. If you know the person very well and you really are very sure this is the best thing for them, and you aren't going to skip away from the flak, that's also one thing.
But unilateral decisions that change someone else's life when you have no connection and don't know what they'd want? Because you personally feel OK with it? No. First, do no harm. It is not your job to go around correcting people's lives as you see fit.
Oh, and...
I understand that not everyone would want to do that because they don't want the responsibility
No, it's not because we are shirking a responsibility, thank you. It's because we think the responsibility here is not to drop bombs in people's lives when we aren't involved or sticking around afterwards. Part of responsibility is knowing when it's not your place.