FMLA is a Federal law. Every company in the United States with more than 50 employees in a 75 mile radius is charged with upholding it. From your description, your wife's employer (I'm not familiar with the company specifically - they don't exist in my part of the country) is certainly large enough to be subject to it. Since your wife has worked for them for more than 12 months, since she has worked more than 1,250 hours and since her medical condition is specifically covered under the statute, she is an eligible employee. This means that if your wife's employer fails to return her to a position that is equivalent in ALL respects to the one she left, they will be in violation of the law. It doesn't matter what the company policy is or whether or not they have those policies written down. As I said, this is a Federal law and they can find themselves in serious trouble if they do not comply.
This does not mean that she necessarily has to be returned to the exact same store she is in now. They could put her in a permanent position in a store that is 15 or 20 minutes away, instead of ten. But they could NOT put her in a permanent position in a store an hour away, or put her in a floating position instead of a permanent one, since that would be a significant change to the conditions she was in previous to her leave.
If they fail to comply with FMLA, your wife can file a complaint with the US DOL and I assure you, her employer will not like what the DOL has to say to them. (BTW, they CANNOT legally take any action against her for filing such a complaint and if they do, she can amend her complaint to include a wrongful termination/wrongful demotion/wrongful whatever.)
Your wife needs to make it clear to HR that she is applying for FMLA and that FMLA protects her right to be returned to a position that is equivalent in ALL respects.