I know that you can't use Speak to the Dead to an enemy. Surely the dead one is pissed with you for having killed them.
What is somewhat inconsistent, is that you should be able to use the spell while using a character which was not in the fight.
Except, that even if you use a character who wasn't present in the fight, is still not able to use the spell as he/she is considered an enemy.
Since they had "nothing" to do with this death, they should be able to use the spell.... right?