I feel your heartache over this and I'm so very sorry it happened. It's certainly a bit of hell on earth.
After reading every single day on this website about dogs murdering/maiming chickens with all too often the innocently unsuspecting chicken person's own dog sadly the culprit (this is a particular nightmare that I can't fathom enduring, which is why I will not have a dog while I have chickens) - I can't in good conscience generally concur with the idea of getting more chickens while a chicken killing dog (however wonderful in other aspects) is in residence. It's just so hard to have a fail safe system, despite best intentions. Here is a case of a 16 year old dog that none of us would likely have suspected would jump up on the couch, much less be inspired to go on a killing spree, and yet it happened.
In this case, it doesn't sound like the dog will live a great deal longer anyway because of its age and medical problems so my gut is saying to wait on getting chickens until the dog is no longer with us if there is any chance whatsoever that this could happen again. The one confounding thing in this particular case is that now there is only one chicken left and it really will need a feathered friend so this situation may require getting at least one chicken now as a friend for the survivor.
I am not able to think of casualties like this casually (e.g. accidents happen, you can replace the chickens), for it is total terror and torture for chickens that are attacked and/or killed. Those beings can never be 'replaced', really. Their unique and special lives are extinguished. While killing may come naturally to a dog, and it's not their fault, it is on us completely if we intentionally put predator and prey in close proximity and tempt history to repeat itself. Exception - fail safe system, but is there such a thing? We feel the pain we feel upon these losses because the beings we lost are irreplaceable, as unique as each one of us (we would correctly assert that each of us is irreplaceable, even though there are 6 billion of us roaming the earth!).
I wish I had the power to turn the clock back to before that fateful moment to spare you and your feathered friends this terrible event. Your intentions were nothing but good, which is what makes this especially sad. Your hen is going to need a lot of TLC right now - what she witnessed and the loss of her feathered friends - she must be stressed. No doubt you will help her through it. If you have or can get Rescue Remedy it may help her a bit (Bach Flower Remedy - a drop in her water would be fine - health food stores and now many pharmacies carry it). You can take it too if you'd like.
Sending best wishes and again, I'm so very sorry. ..JJ