We live in an old house beside a creek, and keep poison in the attics and cellars all the time. After an unfortunate infestation when we first moved in, that is what we have to do to take care of the problem. I'm sorry to say that your friends have likely been careless about prevention.
It seems to me that her reaction was very mild - like she was accustomed to seeing the rats. Even though we wouldn't want guests to be alerted to the presence of rats, I think that most of us would be very startled to suddenly hear that a rat was in the kitchen. I know that I would probably lose any social "cool" that I had, and reflexively make some very loud noises.
I would not want to eat in that house for a long, long time. Rats carry nasty diseases, and leave their droppings and maybe fleas around. I googled up "diseases carried by rats," and just the search page was sickening, much less reading any of it.
As another poster said, there is probably more than one. I don't think that a week's time is adequate to thoroughly clean that house, providing that the owners even want to do it. If you must go there, eat beforehand and watch where you sit and so forth.