Well, it's not normal for a chicken to cough. In fact, in the almost four years I've had chickens, I've never heard one cough. The usual protocol is to separate them from others and observe and if the symptoms continue, put them down. Yes, I did say euthanize them. SO many diseases make them carriers for life, able to infect others even if they seem to recover. Did you quarantine them from your flock for a month first? If not, if they do have something, they have probably infected the rest.
As far as her saying her other chickens dont have symptoms, I find many folks don't even know their own birds are ill because they do not spend any time observing them or pay attention to the sounds they make. Could be that the first birds you had before you got these are carriers and made the new ones ill, but unless you have them tested, no way to really say. Usually if it's just dusty bedding or too much ammonia they sneeze, not cough.
