Let's review what "employee" contribution means: "while the employee writes checks from their personal bank account for the employee contributions."

That just completely bypassed all payroll reporting requirements. The person made a Personal Deposit, outside of Payroll, which is not allowed. That's supposed to be Deferred from Compensation, when you discuss the Employee Contribution. So, you just went to Personal, not as payroll.

There still is time to amend 20148 for everything this bypassed. You might need to meet with a CPA that deals with Retirement Plan administration, ASAP. The corporate tax return is due Mid-March, not Mid-April.

Here are some articles for reference:

https://www.goodfinancialcents.com/solo-401k-contribution-limits-rules-plans/

https://obliviousinvestor.com/solo-401k-contributions-employee-or-employer/