Questions like this always make me feel sorry for taxpayers who live in Rust Belt states where cities have an income tax -- that usually goes along with high property taxes.  I feel sorry for their tax practitioners also, even if it means extra income. 

You can't tell her not to file, but what happens if she just ignores the city tax?  Is that a city where they withhold the tax anyway, so it's a zero-balance return?  Do they send auditors across the country to track down delinquent returns?  

Grand Rapids (not that one in MN, the other one in MI) -- nice place to visit, don't miss the Gerald Ford Museum, but I wouldn't want to live there.