What does Mrs. Jones say will happen to Roger if he gets the shoes though dishonest means?

After Mrs. Jones has forced Roger to wash his face and eat a good dinner with her, she gives him ten dollars to use for the shoes he wants, the shoes for which he was willing to steal her purse. She seems to feel a great deal of compassion for Roger, despite his attempt to burgle her, because she has "done things, too" that she'd rather not tell him or anybody else, including God, for that matter. She has, evidently, made at least some sort of similar mistakes in her life, and she seems to realize that the best way to teach the young man a lesson is not by reporting him to the police but by treating him kindly, like a young human being who has simply made a mistake. At the end of their night, Mrs. Jones tells Roger,

Now, here, take this ten dollars and buy yourself some blue suede shoes. And next time, do not make the mistake of latching onto my pocketbook nor nobody else's—because shoes come by devilish like that will burn your feet. I got to get my rest now. But I wish you would behave yourself, son, from here on in.

Now, shoes he gets by stealing won't literally burn his feet, but she implies that because he would have got them by doing something bad—something "devilish" (and the devil is associated with fire and burning)—they will have a really negative effect on him. Perhaps they will lead Roger to commit other crimes and do other devilish things, or perhaps they will weigh heavily on his conscience as a result of his dishonest means of procuring them. Either way, the result will not be good.

Mrs. Jones has caught a young boy called Roger trying to snatch her purse. In his defense, Roger claims that he only wanted the money to buy a new pair of shoes. Mrs. Jones figures out pretty quickly that Roger's no serial criminal; he's just a poor kid from a broken home who needs straightening out. So instead of frog-marching him to the nearest police station as we might expect her to do, Mrs. Jones takes Roger home with her, where he can clean himself up and get something to eat.
The old lady senses that Roger's basically a good kid, so she gives him $10 to buy himself a pair of blue suede shoes. But at the same time she also warns him that shoes obtained by "devilish means," as she puts it, will burn his feet. By giving Roger this warning, Mrs. Jones is hoping to scare him straight.