Limit her grazing in the spring/summer. You can put her in a dry lot, a stall or if that is not possible, put a grazing muzzle on her. I have to do this with my minis. Some ppl feel bad muzzling them, but a grazing muzzle DOES allow for some grazing, just minimal amounts and a muzzle is better than the health issues that an over weight horses faces.
Even though she is an easy keeper, she still needs proper nutrition. In this case I would recommend a good vit/min supplement (such as Dynamite) along with an animao acid supplement (such as Tri_amino by Uckele). Vit/min supplements lack amino acids and are a very essential ingrediant in a horses diet.
If she will eat the supplements alone, awesome. If not, you may need a carrier feed to mix in with the supplement. By a carrier feed, I just mean a handful of some kind of feed to mix the vit/min supplement with so that she will eat it. In this case a cheap sweet feed CAN be used. A hand full of sweet feed a day is not going to hurt her. Now, pounds and pounds of it every day can/will, but this is not the case if it's being used as a carrier.