You could make an argument that it would still be right to take the offer, since me and frank will both die after a while anyway.

I expect I still probably wouldn't kill frank though, since: A: I'm not sure how to evaluate the utility of an infinite amount of time spent alone B: I would feel like shit afterwards C: Frank would prefer to live than die, and I would rather Frank live than die, therefore preference utilitarianism seems to be against the offer.