Some times you can't save them all. Its a hard lesson learned. Years of rescues and abused animals has taught me that. You can get the BM involved, but it isn't likely to teach her anything. Its better to educate, then belittle. Try a different approach with her, instead of telling her she's in the wrong, ask her what she thinks of the feed she's feeding? Does she feel its enough for her horse? Then mention something she's doing right (even if its really hard to find), it can even be "I know how much you care for him, so just wondering if you love his feed" or it can be tack, or grooming products, etc.
~*~Never approach a bull from the front, a horse from the rear or a fool from any direction. - Cowboy saying~*~