In the Housing Choice Voucher program ("Section 8"), there's a restriction on renting from relatives. It's at 24 Code of Federal Regulations 982.306(d):
The PHA must not approve a unit if the owner is the parent, child, grandparent, grandchild, sister, or brother of any member of the family, unless the PHA determines that approving the unit would provide reasonable accommodation for a family member who is a person with disabilities.
The owner certifies on various HUD-required forms that he/she is not a prohibited relative. In this scenario, the mom/landlord would have had to falsify those certifications in order to be approved by the housing authority ("PHA").
what are the penaltys for someone committing fraud against housing
In this specific situation, possible penalties for the tenant include termination of housing assistance, disqualification for future assistance, repayment of rental subsidy and fines. Possible penalties for the "landlord" include fines, monetary penalties and disqualification. Fraud charges could be filed against either or both parties.
This is the equivalent of welfare fraud and the penalties are similar. Jail time is highly unlikely, but fines can be quite high.