Him helping out the Grim Reaper kind of makes sense. The two were teammates in the Lethal Legion until the former got fed up with the latter's constant racist remarks, as I learned after Googling for the Man-Ape's current status.
Of course, that also means they now hate each other's guts, but it's hard to fault writers for not knowing that, since it was just one comic from over a decade ago. Still, that's incredible bad luck on G & P's part, isn't it? Of all the C-list villains to choose from, they wound up picking the two who'd never be caught dead working together.