TLDR: do you tell someone that they were being shittalked if you and them already don’t like the shittalker?

I’m a very honest person and I feel like everything is a lot more simple if you tell people what you think about them, but sometimes I overstep my boundaries by telling other people what other people think about them. I feel like when I’m being talked about I wanna know what’s said, yeah?

Anyway, there’s this annoying girl (AG) in the year below me who always comes up to me and tries to be friendly to me but it’s just way too much. She dated a girl in the year below her (YG) back in middle school and they still hold a grudge I guess. I found out they had beef from YG. I’m not really friends with either of them but I’m more friends with YG, who was the one who told me about the beef first. 

Today AG was being as disrespectful of personal space as usual, and she says “you wanna hear some tea? YG? I dated her in middle school. She’s annoying. She hasn’t changed”. I refrained from telling her that YG feels the same about her, but I’m debating on telling YG what her ex said. 

Let me know how you would approach this, since YG and I have kinda bonded over shared experiences with crazy exes and not liking AG. 