Rabbit with incredibly bulged eye, what's wrong?

Posting this for my younger sister. She has a female French Angora that has an incredibly red and swollen eye and we aren't sure what the problem is. It is so bulged out that it looks close to popping out. Other than her one eye, she seems perfectly normal, besides some diarrhea she had for a few days beforehand, but that stopped before the eye issue. Her fur is great, her temperament is normal, her other eye is fine...
What could the issue be? How should we treat it? Is she safe to have around other rabbits?

I am typically not a 'run-to-the-doctor' kind of person, but in this case, I'd be on my way to the vet. Any diarrhea in a rabbit can be lethal, and while that is past, it sounds to me, like there's something deeper involved. Rabbits can go downhill in less than a day. It could just be a bug bite, a piece of sand, fur, or anything - or it could be something much more serious. I wouldn't wait around. That said, if your friend has some eyebright, it *might* help, in the form of eyedrops or while herb, or even a capsule. The trick with bunny's, to getting a pill or capsule in them, is powder it, or open the capsule, and dab up the powder with a tiny bit of banana.

I don't know anything about rabbits, but bulging eyes in humans is often caused by a thyroid issue.  I don't know of anything that can cause it that isn't serious.

If it's just the one eye, it's unlikely to be due to thyroid - that affects both eyes. My guess would be either periorbital cellulitis (an infection around the eye, which can get serious fast), or a dental problem from one of her upper teeth putting pressure on the eye.
She probably needs to go to the vet ASAP, and just in case (especially because of the diarrhea) it would be wise to keep her isolated from other buns.