If my wife lost 45 pounds, I'd leave her alone for the rest of the year. A plateau is a relative thing. It might just be the place where she needs to stop and rest for a bit.
If she's up to it, the next step is obvious to me. I don't think you could push past that without picking up things on the fitness front. The walking may have gotten her as far as it's going to. She probably needs to build some muscle to burn through another layer of fat.