Ooops! Sux to be him... He could have had his character disengage, and run away, also- but didn't. Considering he played his turn- all alone- vs 2 ettins and a bunch of goblins, and chose to attack rather than gtfo, he really can't expect much else.
If you feel THAT guilt ridden about it, I suppose you could give him a mulligan and start up where you left off when he jetted. Unless, of course, the rest of the party finished their combat and moved up. Things will have changed then.
But if someone walks out mid game and mid combat, leaving no instructions, other than oh snap-gotta go! What can you do? Make everyone else wait? Fudging things so the thief doesn't die smacks of favoritism- which would likely annoy the other players. His choice to stand and fight vs overwhelming numbers meant he was 99% screwed regardless.
Question: Did this player eventually return that session?