How to stop a chicken pecking?

I have three pet hens, they've lived together since they were chicks very happily for over a year and are all normally very docile friendly birds. In the last couple of weeks one of the chickens has started pecking another one. I've tried anti-pecking spray for the last week but the situation isn't improving, if anything it's getting worse. They have a reasonable sized pen, 9 x 9 ft and are allowed to roam freely in the garden every day. Can anyone suggest anything? I'm particularly keen to keep both birds if possible, the peckee is the sweetest hen and the pecker is my little boy's hen and if we had to get rid of her he'd be heartbroken. Hope someone can help!