I had not heard this before but it makes some sense. Castor oil is used as a lubricant for dry eyes. Dry eyes in particular can lead to inflammation, which can contribute to cataracts.
Looking in to it I could not find any actual studies, but apparently this is an old remedy used in various parts of the world. But I also read that it can take about 3 months.
It is safe and is worth a try. The only natural thing I have ever seen for cataracts is a dusty miller extract, which is also an old pharmaceutical preparation for eye drops in the treatment of cataracts. But this has to be used very cautiously since it is caustic. So the castor oil sounds like a much better alternative.
If glaucoma, which can also contribute to cataracts, is present then coleus forskohlii and vitamin C sources are recommended to drop the intraocular pressure. Coleus forskohlii will work orally, but again works best for cataracts as eye drops.