Does FICA apply to the situation?  For example, a college student working for the college is usually not subject to FICA.  There are also some occupations that have elected out of FICA for their own retirement programs (it is somewhat common for teachers in some areas to do that).

If FICA should have applied, it is possible the employer was misinformed about the client's Residency status, or may just not have update their records.  FICA needs to be paid through the employer (which could mean amended payroll forms for the employer).

Either way, the clients needs to talk to the employer about (a) asking if their job is subject to FICA or (b) having the employer correct FICA if the job is subject to it.