my guess is the closest one to Disney is just outside Downtown Disney near Crossroads, take Hotel Plaza Blvd like you are going to Crossroads but dont cross straight over 535, take a left at the light & get all the way in the far right lane once you are on 535. Steak & Shake is just past the Sheraton Safari on the right in a little shopping center with a chinese food place, bagel shop, tony romas & CVS or Eckerds (& a coupla other places).
To me, the burgers arent the greatest, like the other poster said, they are pressed very thin, nothing at all like the thick burgers you get in places like Ruby Tuesdays or bar & grill type restaurants. I do love their shakes & especially the malts. They have seasonal flavored shakes right now, eggnog, white chocolate & dark chocolate, in addition to the rest of their flavors, you can also request the shakes to be side by side flavors if you want to pick 2 flavors in the same cup. Their chili mac is pretty good & really cheap & filling.
There is another location on Hwy 192 just past the SuperWalmart & Mideviel Times dinner theater but I think the traffic in this area is horrible.
If your mom likes the old fashioned themed soda shop type of joint, she will probably like Steak & Shake, just keep in mind this is not a dress up type restaurant, they are just a diner. You're looking at prices of no more than say 8 bux for the most expensive combo on the menu. You can order the menu items by itself or make it a combo by adding 2 sides (such as cottage cheese, fries, salad, cup of chili, etc) They serve breakfast all day too.
Fries are great, chili and chili mac are yummy too!
Oh, love the onion rings, too 
