It means someone using and Intuit Product filed a return under that SSN.   

Could have been a Turbotax user that finger flubbed a SSN (is it a commonly used last name?), could be an attempt at identity theft, in any case, your client will need to file on paper, you may want to consider having the client send in a Form 14039 in case it is an identity theft issue.

Since the filing is running late, it is possible your client submitted for their stimulus check by using a "non-filer" portal, instead of the IRS portal for providing Banking info. For instance, TurboTax was letting people file a "basic" or minimum return.