Tell her "Squeeze the abs hard, like you're doing an ab crunch." Then, during the performance of her warm-ups and work sets, just say "abs" to remind her. An exaggerated cue that stresses the opposite of her already exaggerated spinal position should average things out, so that she ends up maintaining something more like a neutral spine.
And rather than touch the lumber spine, you'd want to make contact with the flat of your hand against her abs often while you're coaching her, to reinforce that abdominal recruitment as much as possible.
-S.