sorry for your granny.
I wouldn't trust roos. mine grew up together and recently had a bad fight with a lot of blood. I have a rescue hen I must keep separate from the others. if you can keep flocks separate is the best option. especially if they will not stay with you forever.
Everything is good with her now, but chickens will probably stay with me. Since they will come when I won't be home, anything that my parents do will have to work for at least tonight. For every case, I cleand the coop and pig pen and moved roosts and nesting boxes in the coop and placed food and water on several places.