Interesting question. Let me see....
I gather that the defender who revoked didn't play a trump. I wonder why? It doesn't matter to the ruling, though.
Was the revoke established? (If either defender played to the next trick before dummy asked, then it was.)
Assuming it wasn't established, it must be corrected. Here's where it gets interesting. Law 63B says
When there has been a violation of Law 61B, the revoker must substitute a legal card and the penalty provisions of Law 64 apply as if the revoke had been established.
61B speaks to who's allowed to ask whom about possible revokes. Since dummy isn't allowed to ask defenders, 61B has been violated. That law doesn't seem to care which side the asker belongs to. So now the penalties of Law 64 come into effect, even though the revoke wasn't established.
If the revoke was
established, then it can't be corrected, and the penalty provisions of Law 64 should be applied. So it looks like this revoke rates a penalty whether it was established or not.
We haven't dealt with dummy's infraction, though. It is subject to a procedural penalty under law 90. In this case, I think I'd be inclined to just make it a warning.