She gets them custom made. I don’t really care if someone sells an X cup, per se, I’m just saying that that would be the cup size, would it not (for the US)?
Based on basic measurements (a difference of 24” between underbust and overbust), she’s in the cup size category of OO (UK sizes) - but there isn’t an OO bra available. Based on a direct conversion to US sizes, that would come out as an X cup (which doesn’t exist).
However, direct measurement and consulting a chart (if one existed in the real world for those cup sizes) isn’t how to fit for a bra. You have to take into account her boob shape - and also her leaning and lying bust size - and her back size.
I’d be more inclined to think she would fall into a Q/QQ cup size (UK) - which is theoretically higher than a US X cup (based on a basic conversion).
But I think it’s all moot anyway. Let’s just say she’s big - and there’s no commercially-available bras to fit her exact shape and size - so she wears custom or modified bras.