I would have first checked the email address that's actually in the booking.

If it's Joely's email address, then something has gone wrong in creating the booking or originally inserting the email address into the booking, but the booking update email has gone out as expected in the circumstances.

If the booking has no trace of Joely's email address, then there's a potentially more serious gremlin in the works that's perhaps randomly taking emails intended for address X and sending them to address Y.

I would have thought the ICO wouldn't be terribly interested in an accident of the first kind. Stuff like that happens; that's life.