What doesn't make sense is that if someone "gave up their seat" they are by definition a VDB, not an IDB.

It sounds like a bad case of "telephone". Is it possible that the OP's friend just didn't have a seat assignment and that the person who "gave up their seat" was actually a volunteer for VDB? If so, the story would make sense, but there wouldn't be any IDB.

ETA: it would also explain why there was no apology, as none would be needed (since the VDB pax would be getting a voucher and the OP's friend would be on the flight, presumably everyone would be happy).