A have a friend who is self employed and her earning are only cash how can she get social security. She files joint return with her husband.
Self-employment earnings are required to be reported on Schedule SE of a person's income tax returns. The self-employed individual must pay the appropriate self-employment (i.e. Social Security) taxes, and those earnings are then credited to their Social Security earnings history the same as are wages from an employer. If a person fails to properly report their self-employment income or fails to pay their self-employment taxes, then they receive no credit from Social Security for those earnings.
However, even if your friend doesn't end up qualifying for Social Security benefits based on her own earnings history she may be able to qualify for spousal benefits on her husbands record. In order to qualify for spousal benefits her husband would have to be drawing his benefits and your friend would have to either be at least age 62 or have a child in her care who qualifies for child's benefits and who is either under age 16 or disabled (https://www.ssa.gov/OP_Home/handbook/handbook.03/handbook-0305.html).