Goodness! Some children do present their parents with unusual requests. A young girl wanted to learn double bass the other week.
Now xylophone! Prices go from £300 to £3K or more, but for a beginner I wouldn't consider it a very 'satisfying' instrument. Why did she give up piano, and how far did she get? Not that far, I guess, if she was learning pieces from memory.
I coached children in primary school, when I was a TA, having recorder, keyboard, and percussion clubs, over a period of twenty years or more. But they were always 'fun' sessions, not for Grades or serious study.
Why do you say she HAS to do some kind of music? Is this for her benefit, or for your own ego? Sounds to me like a sure way to put a child off music for life.
Personally, much better I think, would be a good Keyboard, or digital piano with some Keyboard features. That way, you get a range of sounds, accompaniment styles, facility to record and multi-track (either self-contained in the instrument, or via MIDI and computers). And a Keyboard will have a range of xylophone, marimba, vibraphone sounds, that could be played with strings or choir backings.
Many secondary schools do music, but also music technology, where students learn about recording, composing, orchestration, and modern features that I haven't caught up with yet!
I have replied on many music related topics, so if you 'search' my name, and piano, keyboard, music etc you will several other replies.