I have very little sympathy with cat owners who allow their cat to be exposed to the dangers of the world.
Though I feel for the cat (and maybe a little for the irresponsible cat owner), I think that you personally need to think about your dogs and their well being. As has been mentioned, you can't be certain that the cat didn't obtain life-ending injuries before or after your dogs attacked. As such, your dogs may very well not be responsible for the cat's demise.
I agree with the suggestion about making notes regarding the incident, but I would not attempt to make my dogs publicly liable for the event.