Primarily, I would worry about chicken wire not being strong enough to hold back a determined canine. And the birds will definitely become stressed. But if this is your only option, I would make certain that the dogs are unable to dig into the pen, because they will try to get in any way possible. The cardboard seems like a good idea to remove some of the visual distraction. But remember, dogs are led by their noses - they will be just as interested knowing that there are birds behind the cardboard, even without seeing them.
A dog that has not been raised with chickens can certainly be reconditioned not to view chickens as prey but this does take time and a good amount of focused attention. Any attention toward the birds must be corrected immediately. We acheived success with a wild dog but it did not happen overnight. The dog we have now is full time part of the flock but he has been with chickens since puppyhood and is a Maremma Sheepdog, bred to guard livestock.
Another problem that you might have is that most dogs owners, even if they are closest friends, do not respond well to other people correcting their animals. Many dog owners treat their dogs as members of the family and forget that they are dogs, not humans. They may get really touchy if you try to recondition their beloved family member. You certainly don't want to stress a friendship. I have close friends who have a hyper dog who jumps on everybody, is all over the furniture, and is basically uncontrolled. When I am there the dog knows immediately that I am not to be fooled with and will lie down or leave the room quietly. I have never used any methods other than an intense look, a grunt of disapproval, and, at last resort, a quick touch to the neck. Although this is not causing any harm at all to the dog, my friends, while they joke that I am the Dog Whisperer, are still noticably uncomfortable and worry that I am hurting the dog's feelings or self-esteem. Nonsense. Some people will never understand how different a dog's mind works than a human. But the fact is this: If the dogs you are dealing with are not corrected immediately and with absolute authority it may only be a matter time before an incident occurs.
I hope that these friends of friends are willing to let you spend a good amount of time with the dogs. Otherwise make sure that enclosure is very secure: MAN'S BEST FRIEND IS A CHICKEN'S WORST ENEMY.
