Coming back from holidays with family can give you some interesting or wild stories to retell about how ridiculous your family members are. In the midst of recent racial tensions and a terrifying election, people also use their time away to establish their white allyship. White people love telling you about their racist uncle or racist aunt and that unbelievably offensive thing they said like it's some hilarious joke. They use it as a way of letting you know "hey I'm not like that, but I'm going to relay this super insulting statement that my family members said even though it's probably going to hurt your feelings."
This can totally be coming for a good place, but guess what, I really don't want to hear what your racist Great Aunt "Susan" said. It doesn't make me feel better and it doesn't make me like you anymore. I don't want to know what that person said unless the story ends with you calling them out and shutting them down. If someone says something offensive and that's it - what I'm getting instead of you being an ally is you being an observer.
If you feel the need to tell me what someone said and it was against my personhood, I also want to know what you did to defend me and the people like me who've been offended. If the story is just a recollection of what that person says, it means that you sat there and watched it happen, and honestly, as my friend or acquaintance, that's even more offensive.
Of course, this isn't taking into consideration that you should feel comfortable calling someone out. If you don't feel safe in a space-correcting people then you're not obligated to put yourself at risk. But the least you can do is keep it to yourself unless you're bringing up the interaction for discussion and why it's wrong and what you wanted to say but couldn't. It shows more allyship in knowing what was messed up about and what to say than not saying anything and just writing it off as an interesting conversation starter with your marginalized friend.
The moral of the story is that it's not cool to repeat offensive things to the person it's aimed to offend if you didn't correct it or are opening up a discussion about it and what was wrong with it. We get enough of it out in the world, to hear it coming from a friend when they haven't corrected the problem or acknowledged what was mess up about it can hurt. So just think before you talk and take into consideration that no one wants a rehash of your racist/homophobic/classist holiday unless you're metaphorically (or literally) punching the bigot in their hate-spewing face.