Help running out of time!

I’m posting anonymously because this involves my best friends darkest secret, but I need Help! I couldn’t post otherwise.

So my husband’s sister is getting married Saturday, to this complete asshole of a prick. I say this because we all went to school together, and he was a giant control freak, misogynistic, asshat. From what I can tell nothing has changed.

When my bestie was a freshman she got knocked up by this dude. She never told anyone who the father was. She aborted it and didn’t feel the need to share.

Last week on girls night, we got super drunk and she told broke down crying and told me everything. Including how last October (when he was engaged to my sis in-law) that she and capt. douche slept together again. It’s less than a week before the wedding! What do I do? I don’t particularly like my sister-in-law but I don’t want her in a miserable marriage, doomed to fail. How can I stop her with out telling my best friends secret. Am I a bad person for not saying something right away? I don’t know what to do. I wish my bestie had never said anything. Please help!

Replying to Asa*

I told my husband as suggested,”he said it’s her mistake to to make, he warned her about him”. He also figured out it was my bestie.

Replying to Buraks Mom/Update**

I don’t have to like her, to care for her. As you said she’s family. That’s why I’ve been agonizing, I personally would want to know. I would also rather get this information from a friend. My husband and I have decided leaving a typed anonymous note on one her bridesmaids cars some time this week, with enough info to help. I’m sorry if this seemed “immature” to you. I only want her and our family to be happy. I only wish I learned of it sooner. Thank you ladies for the help.

Final update**

The note was delivered successfully, no was the wiser about where it came from. The made of honor, brought the note to my mother in law. And everyone was called to a pre-rehearsal dinner. The Groom flipped and denied everything, my sister in-law called off the wedding. I assume all issues were resolved because the wedding was called back on. I have no idea what he said to her, but the family is still pissed about the note and my husbands dad did not walk his daughter down the aisle. Thank you ladies for your wonderful advice in this difficult situation.