First of all , I don't think this is theft. That being said, it seems strange to me that he didn't take steps to transfer the coins to someone else after he got sick. I'm not sure how he died, but if people like you knew he was very sick beforehand, it seems a bit irresponsible to have made no plans for what unfortunately ended up happening.
Some people think about themselves and friends and family first, not about damned money.
Leave the guy alone. Even if the Bitcoin were stolen, it is not any of your concern.
Anyhow this is up to theymos. He decides how and who keeps the money.