There are a few possibilities here: First, it's possible that the USCIS has information that suggests that she worked unlawfully; second, it's possible that they sent the RFE based upon a mistake or misunderstanding on their part; third, it's possible that the husband or wife made a mistake when completing immigration forms that suggest that the wife earned income while inside the U.S.

There is no way to clear this up in the abstract, so it would be advisable for the couple to get help from an immigration lawyer.