As the others have said, I really hope she's on pasture the majority of the time and that 1 flake is to SUPPLEMENT her. Horses should always be continuously grazing or you'll going to run into all kinds of problems like ulcers and colic just to name a couple of the main ones that come to mind!
Back at my old barn that had grass pastures, my mare got 2 large flakes in the morning, then out to pasture all day, then 2 more large flakes when she came in at night (they were more like 4 and 4 flakes since they were so large). And she is an easy keeper and still didn't get a hay belly from that. Granted, she's also an Eventer who's being ridden for at least an hour 4 times a week but still!