It's called Worker Misclassification:

You must file Form 8919 if all of the following apply. You performed services for a firm. You believe your pay from the firm wasn't for services as an independent contractor. The firm didn't withhold your share of social security and Medicare taxes from your pay.

In other words, your taxpayer didn't get Wages. That's why nothing is withheld. An independent contractor is responsible, as they are running their own business, for all of that. Or, the arrangement was a lie, and the taxpayer was working misclassified.