Columbia handled the protests really poorly. The problem is that Trump wants to use that to attack universities he doesn't like. He doesn't care about antisemitism a bit.
Now, if Khalil had committed a crime, okay. He could lose his green card.
if the university had taken action and expelled him, he could have lost his student visa (he switched visas in the fall). All that would be fine and within the rules.
but the government isn't even arguing he committed a crime, merely that he "supported terrorism" and is against their foreign policy, and that's not within the law. They deliberately sent him to Jena which is known for being very tough.
it is worth noting that the group he was with, Columbia University Apartheid Divest, did applaud Hamas and Hezbollah and also did study sessions on the PFLP. But Khalil's views are being conflated with CUAD. They aren't presenting evidence that he personally supports terrorism, and traditionally, mere verbal support has been considered free speech and protected. People have been prosecuted or deported for material support of terror groups, eg money or housing terrorists.