Your friend should have turned him in for applying for a job with a fake SS#; your friend is lucky he wasn’t penalized as an innocent player in it.
The Immigration Reform Control Act [IRCA] of 1986 makes it a CRIME for the employer NOT to notify the authorities [once the illegal status is known].
While he could possibly be found NOT to be liable for prosecution PRIOR to knowing the man is illegal - he IS NOW liable for knowingly employing an illegal.
If ICE raids his place - he will be in deep kimchi. Better he drop a dime on his employee [the Feds might decline to prosecute your friend], or get rid of the employee all together ...