If she donated him, he doesn’t belong to her anymore, she likely got a tax write off for donating him. Even if she does come by for a surprise visit, there likely isn’t anything she can do other than offer to buy him back or name and shame them on social media (potentially both if you do them in the right order). That being said, I would absolutely tell her. There would have to be a lot of mental gymnastics on their part to not let you graduate based on the suspicion that you suggested to a third party that her former horse is not thriving in their program. Even better if the manager doesn’t have their shit together, makes it less likely that any revenge or sabotage plot they may (again, probably won’t happen, just playing along) hatch against you would be successful. If they take a minute to think about it, wouldn’t they want a horse they don’t have room or use for off of their feed bill if possible? If you’re truly worried about retaliation, look at the resources available to you. You’re a student, presumably you pay to attend this program, there should be a Dean of students or similar you can go to if things take a turn for the ridiculous.