Round 684: Ethel does not answer. Marty sees her appear at 35N15E, still blinking in the fog, and breathe. Marty dodges partially (rolled 26 Ref save), but still takes 20 points of force damage. Marty has 21 HP remaining.
Marty hits once (natural 20, crit confirmed with roll of 25; other attack missed because of fog) for 25-10=15 points of damage. Ethel has 135 HP remaining.
Round 685: Ethel moves south out of the cloud to 25N15E. Marty misses with AoO (natural 1). Ethel stops blinking.
Are you counting movement along the ground as part of the minimum forward speed? I don't know what Ethel's maneuverability class is, so I don't know if she has a minimum forward speed, but that seems like the only reason to move southwest first.
Regardless of Ethel's location, Marty moves into the solid fog first to 50N5E25Z, then to 50N10E25Z -- slowed to 5ft-speed from the fog. As given, he'll provoke an AoO on his first movement.