I wouldn't freak out. I'd store the disk with the same amount of care that I store my own disks, and I would insist that it is 100% encrypted with free space either encrypted or wiped. (If theft of a powered-off storage medium compromises the data, you're doing it SERIOUSLY wrong.)

Someone who is storing disks offsite is unlikely to have only one copy. I'd offer to review his backup strategy with him, but other than that, I'm "dumb storage", how my friend uses it is his business. I'd also make clear what expectations are if law enforcement or someone with a gun comes for the disk (they're getting it, that's what the encryption is for and I'm not going to jail/dying for something that should be solved with replication).