If she has access to grass that is probably why she doesn't want to eat the hay. I know mine won't touch hay if they have the option for grass so I wouldn't worry there.
I also have to disagree with Tremor on the Purina feed quality. I hate Nutrena with a passion, the only time I've had issues with colic and choke in almost 15 years with minis have been with Nutrena. Now if you have access to Triple Crown, which I did for a very short time, I would suggest their senior feed - its just wonderful.
I feed Omolene 300 along with Strategy and really like how my horses look and feel on it. If someone isn't holding weight, just up the grain a bit and that fixes the problem.
I had brought in a BAD case of malnutrition in a 20+ yo mini mare, worked her up to 2.5 lbs of omolene 300 grain twice per day and in two months she was beautiful.
Not every grain works for every horse and neither do the amounts. I have a soon to be gelding that can't have any grain whatsoever or else he gets a very very bad case of the runs. I'm thinking he is allergic to corn so I am putting him on a oats and alfalfa pellet diet along with a weight supplement (he's a bit harder to hold his weight) and free access hay.
The thing is, is to find what works for YOU and YOUR HORSE. Ask 100 different horse people what to feed and you'll get 100 different answers!