Question came up this weekend. Two opposing ships at Pilot skill 9 (Fenn Rau) attacked simultaneously. The ship with initiative attacked Latts Razzi (PS 5) in his YV-666 and destroyed him. The other PS 9 ship attacking a Jumpmaster wanted to use Latts Razzi ability, that triggers when a friendly ship declares an attack, to reduce agility of the Jumpmaster. Was Latts already destroyed and not able to assist?

Correct.

Simultaneous Fire ONLY keeps a ship on the board if it has the SAME pilot skill as the ship that destroyed it AND it hasn't yet activated in that combat round.

Check page 3 of the FAQ for the most recent Simultaneous Attack Rule.

It helps if you remember that the "Simultaneous Attack Rule" is badly named. It's not actually simultaneous. It doesn't take a snapshot of the game state or perform magic time-travel. It should really be called the "Parting Shot Rule", because it only does ONE thing: if a ship would be destroyed, AND it's the same PS as its killer, AND it hasn't shot yet, then that ship stays on the table until it has shot. All other game state proceeds as normal.

So, in your example, Latts is already destroyed and can't assist. In fact, in the scenario you described, the "Simultaneous Attack Rule" isn't even triggered at all, because the ship that was destroyed wasn't the same PS as the attacker.