I doubt there's any specific law that makes him speaking at the university "allowable" other than the first amendment itself.
I mean. Not specific to universities, but free speech has been upheld in courts under first amendment grounds for decades and decades.
What I'm saying is, there needs to be an amendment to such legality to exclude hate speech that specifies race, gender, nationality or sexual orientation. We don't currently have that and won't have that for probably our whole lives, so until then we must fight these genocidal maniacs with everything we have. Which is protests, shouting, and sometimes a good ol' glitterbomb.