mamakim I really think you should mention it to her, for two reasons ...
You say she's very young.
Firstly, as she's now left the group it is possible it was a mistake of some kind, and doesn't actually reflect her views - e.g. it's possible that someone she knows joined her as a joke, or out of spite, and she left when she found out she'd joined it. I know a group of young people who are constantly doing this kind of thing to each other - not racist stuff, but signing each other up to all sorts of embarrasing things if one of them is careless enough to leave themselves logged in to FB etc, or to reveal their password.
Of course it's very probable she joined the group deliberately, but you won't be able to rule this out unless you ask her.
Secondly, as she's young, it's now that her opinions are being formed. Especially if you've been getting on, it's important that she knows that you don't find these kind of views acceptable. Her views may change over time. I gained some new family about 15 years ago. When I met them they were in their early teens. I really liked them, and was very disappointed to hear them express racist opinions. About 5 or 6 years later I overheard then berating someone for being racist, and I commented that their views had changed. They told me they'd grown up, basically, that there were only white people where they grew up, and they had gone with the flow, as they hadn't known any better. As they got older and had more experience of life they saw sense, and are two lovely young men now without a racist bone in their bodies. It helps that their family are not at all racist - would it be worth metioning it to her family? Have you spoken to your mum about it - perhaps she can shed some light on it?