There are posters here with a lot more insight in this area, I'm sure, but do you think she'd believe the girl herself? Maybe if this girl called to apologize to your W for her inappropriate comment, it would be appreciated. No need to mention or apologize for going out on her part, I wouldn't think.
Another option: ask your W what would make her feel comfortable again. You guys both dated while S, right? So what would SHE now want YOU to do to show her that you are serious about HER, not anyone you were with then.
Nicola
Life isn't about finding yourself; it's about creating yourself My thread: Trusting God's Plan