This type of event has been discussed in detail before.
Keeping the autopilot connected is important, because of the reduced longitudinal stability at high alt.
As an aside, exactly the same thing has happened before in my company on a big airbus over the himalayas. Same thing (tho they were in level flight). The skipper was also out of the flight deck on a comfort break. FO got into alpha prot and ended up descending a couple of thousand feet (triggering a TCAS RA on the way down).
Disconnect the AP at your peril.