I think the root cause of the problem is the unclear policy by FB. Privilege escalation can be hard to catch, and can be a separate bug in and of itself, even if it requires a separate exploit to get the initial privileges.

The published policy didn't say anything about not doing what he did. I'm not going to argue that what he did should or shouldn't be ok, but FB has no control over what other people do. Yeah, maybe it'd be better if people asked for clarification first instead of asking forgiveness, but there's no way to force them to do that. FB does have control over what their policy says and allows/disallows. If you don't want people to exfiltrate any data and look at it on a local machine instead of just keeping a session on the exploited machine, then put that in the policy. If you don't want people poking around for other exploits after gaining access, then spell that out in the policy.

The point of the policy isn't to stop everyone. Sure it will stop some/most people, but some people don't listen. The point is that when it happens again you can point to the clear policy and say "you're an asshole, we're not paying you because you violated our explicit policy, and we are reviewing what you did with our lawyers to see if we should notify law enforcement".

Yes, doing this fix/policy update now doesn't fix this situation, but it prevents anyone else from doing something similar and claiming ignorance of this situation and FB's position.