If she's putting food out in the field, and she thinks he's eating it, tell her to move it a little closer to the house each time. And I agree with Tina, tell her to just sit quietly and let him scope her out. He may realize that it's mama, but don't reach out to him cause he'll just bolt. The sound is a good idea too, I have a specific sound for my cats, not the typical "meow, here kitty kitty", and they always come running if they hear it. Good luck to your friend, tell her to be patient. Oh, and since it's so cold maybe put out fresh water too.