Although the tenants were "out" and handed back the keys, they've been returning at night every few nights -- I think they are still getting stuff out. There was a LOT of pot growing things around. I stuck my head in the back house (the sliding door was open), and the smell was awful.
Anyway, I talked with the owner today as he was surveying the damage. He says that he is considering "giving it back to the bank." There was a reverse mortgage, and a lot is owed. It's a real mess, and hopefully it will be vacant for a while.
I'm not going to blow the whistle at this point based on the advice above. Thanks for the tips.