Nothing will happen. He is breaking no rules.
I have dealt with this too, it is an awful thing to experience, and I’m sorry it happened, but it’s something you must prevent. No one will punish these people, because of legalities.
For reference, here is the profile of a player who has done this since launch to hundreds of horde matches.
Hundreds- thousands of reports, no penalty.
You can tank their reputation, but they will never get suspended or banned for it.
