I need advice!, Sorry guys, I hate to do this. Options

[MissFits]

Okay, I have never done this in my life. I feel like such a dork.

My fiance and I are unsure how to handle a situation we recently encountered. It's kind of a long story and I will try to shorten it for you as much as possible.

John, my fiance, has dated other people before me. He dated a girl named Lydia for a short period. Lydia sucks, I tried to like her but I never could. After my dad died she was angry at my friend so she came to me and was talking shit to me about my dad. Needless to say I HATE her for that, I don't mind that John has dated her but talking shit about my dead father is unnecessary by all accounts. Because of that and several other things we have had our spats.
She and John had broken up before this (it was a mutual breakup, neither of them were exceptionally happy in the relationship). That same day she had sex with one of our friends. John NEVER did anything to disrespect her and was never mean to her in any way.

Fast forward over 2 years. We see her at a party 2 nights ago with her boyfriend Nick, who used to be cool with John. I was nothing but overly respectful to her, as was John. Nick and John spoke, but Lydia didn't say a word to either of us or anyone we spoke to.

Last night several close friends of ours said they saw Lydia pull nick aside when we arrived at the aforementioned party and tell him to beat John up.

Nick didn't do anything, but but the fact the she would want to harm John when he did nothing to her makes me so angry. I can handle when someone wishes harm upon me, or threatens me, but to do it to someone I love makes me absolutely crazy. It makes me SICK to hate someone so much.

When I first heard I wanted to find her wherever she was and just beat her silly. The more I thought about it though I think the more mature thing to do would be to acknowledge that I know what she tried to do and forgive her for it. John, on the other hand, thinks we should ignore it completely.

What do you think?
Sorry it was so long