Yes, Halo 2 would have infringed on Trent and Xavier's game if the patent on Hallowed was released prior to the release of Halo 2 and Halo 2 creators attempt to acquire a patent.
Trent and Xavier do not have the right to sue the makers of Halo 2 due to the lack of information provided. Halo 2 creators have no effort of infringing upon Hallowed because Halo 2 was released before hallowed. Trent and Xavier could try to sue them for copyright infringement only if Halo 2 creators started to work on the game after Trent and Xavier worked on Hallowed but