I ditto Donna S. I was thinking that you could buy a product to eliminate shoe odor, or even baking soda maybe (I don't know about that for sure), and when she comes over, try to be nearby when she takes her shoes off and you can approach her in a joking way. Like "Whoo, that's a stinky shoe, girl!" Obviously with a smile, and more importantly care and concern, and then offer her a way to fix it. "Oh, I have some stuff that will really help with that. How about we go get it and give it a try?"
You could even then go on to mention that wearing socks would help, and be more comfortable.
She may not notice it, or she may actually be self conscious of it, but unable to remedy it without help.
I also agree with Christy, why not throw them in the washer, then set them outside on the porch in the sun to dry.
I think you've got this covered! ;)
Best of luck!
KATIE