Tax avoidance is legal because you use the rules to lower what you owe.
Tax evasion is illegal because it hides income or lies on a tax form.
For a full-time job, you can lower taxable income by saving in a 401(k) or traditional IRA, paying into an HSA, and using deductions.
If a friend hides income, tell them to report all money, talk to a tax pro, and stay legal.