I think you may want to reword your question. What is your clients situation?
Income limits for social security recipients under full retirement age can be found on SSA's website.
If you are talking about "earning limits to receive full SS benefits" for 2019 it is
under full retirement age: $17640
year of full retirement age: $46920
full retirement age: no limit
Are you askin Who must file?, I always go with When in doubt, file. The statute of limitation does not start to run until you file. Filing helps to prevent someone else using your SSN to file a fraudulent tax return.