Getting paid in cash shouldn’t matter. As long as they take out the deductions like SS & Medicare. Do they give him a pay stub showing any deductions? He better ask them if he will get a W2 or 1099NEC at the end of the year for his pay. If they didn’t have him fill out a W4 did they give him a W9 to fill out? They need his ssn.
If they pay him in cash and pay him as a 1099 independent contractor that is different and could be a problem. Even if they don't give him a 1099NEC he needs to keep his own records of the income and report it as self employment income on Schedule C in his tax return. He will pay Self Employment tax to cover the SS and Medicare. He should send in quarterly estimated payments to cover the tax due on it if they don't take out any withholding.
If he has to file as self employment, he is considered to have his own business for it. The people or company that pays you is your customer or client. He can also deduct any expenses for it on Schedule C.
Here is some IRS reading material……
IRS information on Self Employment
http://www.irs.gov/Businesses/Small-Businesses-&-Self-Employed/Self-Employed-Individuals-Tax-Center
Pulication 334, Tax Guide for Small Business
http://www.irs.gov/pub/irs-pdf/p334.pdf
Publication 535 Business Expenses
http://www.irs.gov/pub/irs-pdf/p535.pdf
You pay Self Employment tax on $400 or more of net profit from self-employment in addition to any regular income tax. You pay 15.3% SE tax on 92.35% of your Net Profit greater than $400. The 15.3% self employed SE Tax is to pay both the employer part and employee part of Social Security and Medicare. So you get social security credit for it when you retire.