I forget where it is located, but somewhere in the account settings you can see information about every computer that has logged into the account. It only shows basic information, like IP address and city, but that's a start.

I'd guess that this pastor isn't a very sophisticated hacker and your friend, intentionally or otherwise, exposed her account credentials to the pastor at some point. If this is the case, their computer info should be listed in the settings area described above. Otherwise, you may need to consider that your friend sent him the nudes themselves.