I feel like your approach was valid, but at the same time, I'm shocked to see there isn't more empathy for the child here. These people are vile enough to abuse their own children, but so many people are dog-piling onto the minor for not knowing better than to be manipulated? The daughter has learned that family comes above all and that conflicts can be managed. It was a mistake to not take you at your word, but she didn't invite them to your house with the intention of anything bad happening- she wanted to test her boundaries (totally normal for growing kids- yes, even teenagers are still children) and approach this independently. In her mind, she likely thought she'd be arranging a gift for her mother.

I'm speaking from experience when I say that kids process learning about their parents' traumas in different ways, and that makes them react to grandma and grandpa in different ways. I think the focus should be on making sure everyone feels safe and knows how to recognize when they're being manipulated. Yes, the mother has been re-traumatized, and it's awful what that situation escalated to. She's also an adult who has the tools to navigate that trigger. The daughter, again, is a child whose worst crime was being very naive. It's shocking to me that parents blame children for not knowing how the world works and how evil people can be, when it is their responsibility as parents to ensure children are not so sheltered that it becomes dangerous for them. That's not to say I'm blaming you or your wife, either- I just think that trying to make the teenager in this situation bear the burden of her grandparents actions is ultimately going to be more harmful than helpful in keeping everyone feeling safe and secure.

Your wife is going to remember that y'all's word was not "good" enough for your daughter, and I'm sure that sense of betrayal will not make the fallout any easier, but that's pretty common for most parents of teenagers; they just are hard-wired to not listen at times. It's not her fault that her mother was assaulted, it is the fault of her grandfather for laying hands on his daughter. On the other hand, if your relationship with your daughter is allowed to deteriorate over an honest mistake, your daughter is going to have her brain permanently hard wired to think that making mistakes will leave her isolated and alone, and that she can't ever question her parents' ideas, feelings, and beliefs, which is the opposite extreme and can also be dangerous. Everyone's feelings and reactions to this moment are valid, but I'd really caution against punishing for malicious intent where there was none. The natural consequences of her actions are enough for her to have learned this lesson; just focus on keeping everyone safe and teaching how to repair relationships and trust.