You would invite her to dialogue in a respectful, empathic and caring way. If she is unwilling to do it, then you could send her a letter telling her, with the same assertive approach about these concrete proofs you have, explaining that you choose to take this initiative because of the seriousness of the situation and the evidence you got. You would tell her that you totally respect her choices and what she finally does with her relationship and life, but wanted to make sure you did your best to support her.
If after you do this she chooses to disregard you and your support, then you would continue to respect the boundaries she has set. Letting her know what you know, showing such proofs does not mean you are trying to manipulate her or choose for her, but you just want to be responsible and caring informing her of something real you have proof about.
She is an adult, and she chooses to disregard it, that's her choice and she would have to learn from further pain, and as frustrated and sad
as it ma sound, it would be necessary for her to mature and grow as an individual, parent and woman too. You would continue to be there to support her with your guidance and unconditional affection. That's the best you can do about it.