The previous owner said that this happens every summer no matter what is fed to her. I don't know if I want to believe this over the health of my horse. This isn't that kind of thing that horse just have.
I would take a bit of the manure and put it in a bucket of water. Let it sit for about 30 minutes and then slowly dump it out. If there is sand or dirt on the bottom then yo have a sand issue and will want to consider adding pselinium (sorry about the spelling) to the horses diet. I would suggest sandclear.
I also would consider trying some yogurt (the kind with the bacteria in it) for a meal or two. The probiotics are not a bad idea.
If the teeth aren't that great then I would look into a senior feed. I know 13 is not old but if there is an issue with the roughage intake then it won't hurt to switch to a full source feed. Or to add a hay additive to the diet. But the sand is my first thought on this.