What exactly is a ridesharing carryover?  Is this IRS, or a state issue?  

Did the wife file her own MFS return in 2016?  As @Just-Lisa-Now-  points out, it's probably not worth contesting such a small amount with IRS.  That's especially the case, if she can't explain why she didn't file her own return that year.  But if other issues come up that require a trip back to divorce court, you can add this one to the docket.