Remember to keep this entirely consensual; if she says "no", that's what it means. If she says something hurts,
STOP.
I would try for a water-salt solution. Why salt? Because plain water screws with one's electrolyte balance and one could
DIE from it (
LINK). Please do your research before even attempting something like this.
Try that if she's willing. I'd also invite you to look at...enemas. Again, the above warnings apply. Just a thought. I don't know about food ideas as most of the foods out there, consumed in sufficient quantities, will make one fat.