"He had few 1099-NECs(Total $121000)  under his name and two 1099-NECs (Total $51200) under his S-Corp."

Since an S Corp doesn't even get issued a 1099-NEC under most conditions, this person needs some guidance on how to use Form W9. Also, the 1099-NECs don't qualify as the only business revenue; these are the only customers whose payments to your client meet the reporting threshold. Did you ask about bookkeeping reports? 1099-K?

"In 2023 around June he contributed $6000 on ROTH 401-K and $16500 on individual 401-k under  EMPLOYEE CONTRIBUTION"

401(k) accounts require a Plan. There would be an employer Plan filed that qualifies there to be 401(k) accounts. That's different than when an individual establishes their personal IRA account; IRA accounts don't get established because of a plan.

"( I mean description on Vanguard transaction confirmation ) not under employer contribution."

And, as noted, an Employee contributes from payroll.

"How do I enter on his tax return, on S-Corp tax return or on individual 1040?"

I wonder if this already disqualifies the plan/account.

"I think it should be under EMPLOYER CONTRIBUTION when I see Vanguard transaction confirmation ! Am I right?"

You're no righter than they are right.

 "Any insight would be greatly appreciated."

Let's remember that 401(k) is based on earnings, so that's been violated. Even Solo 401(k) is based on earnings.

I think this might be a bit beyond your experience. Heck, even though I do a lot of benefit work, I would need to do a ton of research on this. Why not find a mentor?

The S Corp is already late filing, if that is what you are working on. There is time to fix 2023 for most things, even though it might result in some penalties. Some things might be fixable and moot, no penalty.