My dogs have been trained not to go for the chickens.
If you know how to train dogs not to go after chickens, then if your friend is a regular visitor with her pups, do the same training with them. Until then, I would not leave them alone in the yard if there's even the slightest chance they can access the chickens. A barrier that can be moved by a 3-month-old puppy is not much of a barrier. That was a failure on your part.
I can understand your reaction from an emotional point of view, but I doubt it will have any effect on the puppies in the long term, at least when it comes to chasing chickens. The prey drive is incredibly strong and IME the only way to reliably overcome it is to consistently and calmly
teach the dog self-control.
When the puppies chased the rooster, that was its own reward, whether they'd caught him or not--chasing is fun, after all. You got to them after they already caught him, so your correction had little or no (probably no) connection to the chase in their minds. To you, it seemed like a split second--but to the puppies, they caught a rooster, then you took it away, put them down, and then slapped them. If anything, they'll associate the slap with you setting them down, not with the rooster.
In the future, even if they did associate the slap with biting the rooster, you still can't rely on that. Once a dog gets into strong prey drive, they're not thinking about much else. That's why you need to train them to not chase or see your livestock as prey in the first place, not correct them for chasing and attacking.
Like I said, I understand where you're coming from. When my GSD first showed up, part of the reason I was determined not to keep him (LOL we see how that worked out) was because he attacked one of my goats and did some damage to the poor thing--though everyone was ultimately OK there too, after a vet visit and some stitches and antibiotics for the poor goat. I tackled Hector to get him to release the goat and smacked him twice after he'd let go--not hard enough to seriously hurt him or even get a yelp, but enough to scare him. It was IMO an understandable reaction (I loved that goat), but it was also an abusive action because my dog didn't understand why I was hitting him. I still regret doing that, even though I was just in a panic and wouldn't have done that in a more normal situation. So I really do know where you're coming from, but short of actively stopping an attack (if you'd had to hit the dog to get him to release the rooster, for example, and even then there are usually more effective ways), hitting a dog isn't really ever appropriate.
Also, regarding the feeding the dead chicken thing discussed earlier...yeah, after thinking about it further, it's kind of silly to think that letting a dog eat a cleaned chicken would encourage him to kill them in the future.
Like I said, I grew up in a culture where killing one chicken was a death sentence for most dogs, so I was always erring extremely on the side of caution, but yeah...it's also a bit hypocritical since I let my dogs chew on horse hoof trimmings all the time and I don't think that will make them more prone to nipping at or chasing horses.