I suppose though you could say that Estelle's not really a friend to her either. After all, she knows how much Zoe likes Michael. Estelle wasn't/isn't even that keen on Michael and there have been no hints that her and M would ever get it on. So Estelle has no big love or even like for Michael, it's convenient for her to get it on with him, yet she doesn't even baulk or have a moments worry she will upset Zoe.
Now it's true, Zoe has no claims to Michael and it's not like Estelle is breaking them up, but it does come across quite mean that Estelle wraps herself around Michael, when she doesn't even like him that much and not care that she would upset someone who genuinely likes him. It seems very insensitive, even if zoe didn't stand a chance and especially when Estelle keeps telling us what a nice and kind HM she is. But I guess Estelle and Michael are playing their games so that's all that matters.