I used to struggle with this too, perhaps partly because of extreme shyness/self consciousness. The worst thing was that it was called 'playing with feeling' and you were supposed to both feel things and express them, neither of which i could do easily, and both of which I was very self-conscious and embarrassed about. I didn't really know what it meant in terms of music, either.
It was easier in orchestra, when the conductor told us things to do that would change the feeling of a piece - slow down here, speed up there, attack the notes here, smoothly there, very soft and gentle here etc. It wasn't us that was having to 'expose' ourselves and our feelings, but we learned the idea that different elements of sound production could change the overal mood/feeling of a passage Sometimes he'd just give analogies of random things that the music should sound like - popcorn, or waves, or whatever, and even without actually telling us what to do, the image in our minds did help produce the right sound. But telling us actually what to do probably helped more to begin with!
So maybe don't tell her it's about 'feelings/expression'. I still don't think I feel much with music, though I do enjoy it very much intellectually, playing/singing especially, but also listening. I don't get carried away with passions the way some listeners do, and don't come away with the same sort of emotional experience that others in orchestra or choir talk about, but I do still appreciate it in a way I can verbalise.
Instead of 'play with expression', which just sounded so overwhelmingly exposing/panicky to someone like me, maybe get her teacher to be very specific with things she could do a various points - when to change tempo, when to change articulation, etc, without it being a big deal that it should come from her in some way, that she should be feeling something or know how to express that in music. Just some explicit instruction to start with; possibly even copying phrases and seeing how similar she can make it to how the teacher/recording does it - then the teacher can show several different ones and she can try them out, see what she likes, etc. But it turns it into a more techical/musical teaching moment that way, gradually learning what the effects sound like and how to make them, rather than some unidentified 'feelings' to express that can just make some children really embarrassed and self-conscious.