I have always found that learning other languages – providing that they are well and interestingly taught – is great fun. I also believe that you can never begin to understand another culture until you have at least a grasp of the language. I don't think it matters what age someone is when they start to learn another language – I a 65 and going for it – but I think it might be easier when your are young. If Chinese (Mandarin?) is what she wants, then why not? Equally, I don't think it matters whether it's a much easier European language, such as Spanish, or something fairly obscure, like ancient Greek.
Mandarin looks like it might be a commercially useful language to learn because, in spite of the current downturn in the Chinese economy, it remains one of the most important economies in the world. BUT not everything is to do with making money; sometimes it is to do with self-satisfaction and doing something which gives you joy.