Ha! People are always talking about what good tick control guineas are (usually people who don't have guineas, I've noticed!) But the stupid things don't stay where you want them or need them! They wander all over creation, sleeping in trees and getting picked off by predators till they're all gone.

They won't roost in a coop, or come when called, or in general behave in a civilized manner like chickens!

Besides (on a serious note) if Granny can't take care of chickens any more, it's doubtful she'll do better with guineas.