The employer did not withhold Fica and Medicare on employees with visa's.
The employee's did not give information to the type of Visa until the end of the year and no FICA or Medicare has been withheld.
Can this be reported as uncollected ss and medicare on W2's or is the employer liable for all taxes

The employer IS liable to pay all of the FICA (Social Security and Medicare) taxes whether he collects the employee's share or not, UNLESS the emlloyee is exemot from FICA taxes.

If the employer fails to witbhold, he xan legally collect the back FICA taxes until 31 Jan of the following year. After that, he loses legal standing sranding in the eyes of the IRS.

Hence, the employee could in theory quit the job and the employer would be "stuck" with the tax bill.

He may be able to recover the smplotee's share in court, but he would receive no help from the IRS.