Lol, i don't think she is telling the truth somehow. 11? i don't think so. Like rosetinted said, she is going through that 13 year old phase of "how mature/experienced am I?!" when really, everything she is saying is most likely to be a lie, generated to make her sound "big". I'd just tell her she is annoying you, and there is no need to talk about sex all the time, it actually would be quite nice to talk about normal stuff! I would get annoyed with my mate if all she talked about was sex, let alone if we were 13 and most of it was bull**** anyway!
PS bit harsh to call her "fat and ugly" isn't it? This makes me wonder whether your friend has picked up the way you feel about her, and is saying all this to try to impress you, and make you like her more? Just a thought.