I personally would not quarantine for just one hen. But please read the following:
There is a good reason to quarantine anyway...and that reason is- what if the new hens have a horrible disease that actually contaminates your chicken yard- the soil and everything?
The worst case scenario if you quarantine (well except what is discussed in the next paragraph) is that the new hens die after you get them of a disease, or that you have to put them down in order to avoid getting your old hen sick.
There are diseases that result in an asymptomatic carrier status and they might pass the quarantine and still make your old hen sick.
So you must weigh carefully and choose.
You can use a dog cage in the garage to quarantine, is one idea.