Hello, this is Dr. Linda. I hope I can help with your question today.
Just like in people, being overweight can lead to many health problems. My primary concern would be laminitis, or founder, a very painful condition that can affect all four feet. In addition, being overweight puts a great strain on the joints, as well as, the heart. Obesity can lead to insulin resistance, similar to diabetes, & equine metabolic syndrome.
If there is no one there to exercise her, then you need to try & limit her access to food. Hopefully, she does not receive any concentrates, either pellets or sweet feed. To cut down on the lush pasture, I would suggest using a grazing muzzle. This attaches to her halter & covers her muzzle. It has small openings that still allow her to pick at grass all day long, & does not interfere with her ability to drink. I have used these very successfully on my miniature horses.
I know you love Canadian Summer, but you are not doing her any favors letting her get too fat.
I hope this makes sense & you find the information helpful. Please let me know if you have any further questions or concerns.