First let me say, I am so sorry for your loss... I sure hope your one roo makes it... I'll say a little prayer...
I would agree with everyone so far on this blog... If you are certain that you know this person's dog did the damage, I would file a police report with comments about the dog running free and if you see the dog on your property again that you will shoot the dog to protect your livestock (chickens included).
If the dog has killed once, he'll be back again... It will be like a game to him/her and will kill for the sport.
If death to the dog should ever happen while you are protecting your property and livestock, this way you have it on record with the local police and the owner cannot come back on you.
There is a fine line in some states and counties regarding blaming other peoples animals in a case like this.
I read on another blog last week about this same situation. One of the BYC member went to court and won (won big) because the dog was caught with the birds in it's mouth. Another BYC member could not do anything or receive compensation for the loss of his birds because the dog in question could not be proven to have done the deed.
If your heart can take it and you are calm enough to speak with owner of the dog, you can see about compensation for the lost birds.
If you are like me and are in rural area, you could check with the dog pound to see if they can do anything about putting the dog in doggie jail. Maybe the owner will think twice about letting the dog run free if he/she has to bail it out of jail!
Of course, with the way the world works today, you may have build a run and keep your babie in the run during the day while you are away and let them free range when you are home.
I do hope things work out for you and your birds will not have to go through this again... Don't be discouraged...