My flute teacher always talks about 'internalization', as in, you should always know how you want something to sound - it should all be in your brain, your instrument is just the thing that gets what's in your head, out.
He uses this as an example... gets me to close my eyes and is like. 'Imagine an apple. It's on a plate. It's red with some green on the side. It was water droplets on it. It has a little leaf on it' etc. and builds it up from something really simple to something really complex and interesting - and that's what i should be doing with my music
It'd be bad teaching to say 'do this here, then do this there' or get her to copy you or something, cause then she'd be learning how to play THAT piece well, but not how to play well all the time. If you get what i mean, i'm not good at explaining! Also she'd be playing your version, not hers.
Just show her how to pull about the speed - count the pulse as she's playing and get faster and slower and make sure she goes with you, then tell her to experiment with that to find what she thinks sounds best - playing at both extremes, with LOTS of rubato, then all strictly in time, then try to find a medium etc.
E.g. with dynamics if my pupils aren't sure as to where to put them in and what to do, i'll play some different examples and explain why i did what where, then ask them which they thought sounded best/most appropriate and get them to play what they think sounds best - even if it's totally different to what i played. (aslong as it's appropriate!)
Also maybe you could demonstrate the 2 extremes - really rigid boring playing, then really rubato expressive etc. And make it really obvious that the 1st is bad and not nice to listen to, and the 2nd is really good.
Just make sure she's learning how to play musically in general in all pieces - not just learning where to add rubato and stuff to that one piece
Btw i agree with BerkshireMum about speaking to her in monotone and showing that's how she's playing... my teacher always does that and it really works! Comparing it to speech is good... point out that when you're speaking you're actually speaking at loads of different pitches, tempos, etc. really varying it but you don't realise cause it's natural - it would be unnatural to speak all in a monotone etc. and that's how music should be - playing expressively and meaningfully without realising that much
Gosh sorry i go on

Me and my ideas

(too many of them, most of which are useless!)