Are there any supplements that help prevent glaucoma?

No. There are no herbal supplements that i know of that can help prevent glaucoma. The only proven way to prevent glaucoma if you are at risk for glaucoma is to lower the eye pressure. Check out the ocular hypertension treatment study: www.Nei.Nih.Gov/glaucomaeyedrops.
No. Not any that have been shown to prevent glaucoma unfortunately. Glaucoma can be a devastating disease that most commonly doesn't have symptoms early so irreversible damage can occur without knowing it. Important to be monitored by properly trained medical doctors.