Haven't read all the posts, but to answer the OP:
A girl I knew was being really nasty about me behind my back and dp heard about it but didn't want to tell me because he thought I'd be upset (right) and because I would probably never see her again so it didn't matter.
I bumped into her about a year or two later and she was pg, I was all nice and friendly and full of congrats - told her that if she needed anything for the baby I had loads of stuff that she could have and said that we really should get together for a drink or something. She was nice and friendly back and agreed.
Told dp and he was horrified at the two-facedness of her, told me what she'd said about me and then I felt like a total mug.
If I had known what she had said, I would still have been pleasant to her but I wouldn't have gone overboard like I did.
I think you should mention something.