If he does not use editor for some other task after session time out he should get the autosaved draft back when Atto is opened again. Autosave does not use session data.
I'd like to know more... maybe these are stupid quiestions:
Is the draft saving kept locally in the browser?
Can the saving time span be set lower without causing increased server traffic/load?
Sorry - just tested with another browser, and the autosaved text showed up there as well. So, I guess each "save" will cause server traffic?