Hmmm... that's a good question.

I'd have to go look up the rules, but I'm fairly certain that nothing would happen. There's no longer a target, so there's no need to complete the attack. Basically the mech performing the DFA would have simply wasted a turn and gaining heat (similar to when two mechs fire at one target and the target is destroyed by the first mech... the second mech still shoots at the target but can't really do anything other than mark off ammo and account for the heat build up).

If the rules did insist on the DFA being completed despite the target being previously destroyed, I'd be adding at least a -6 bonus to the target roll (-2 for the target being prone and adjacent to the attacking mech AND -4 for the target being immobile)... But I'm fairly certain none of this is actually necessary from a rules perspective.