There are two main implications for your friend. One - criminal. Two - immigration.
First. Not filing a tax return is a misdemeanor. IRC section 7203 states in part that:
[a]ny person required under this title to pay any estimated tax or tax, or required by this title or by regulations made under authority thereof to make a return, keep any records, or supply any information, who willfully fails to pay such estimated tax or tax, make such return, keep such records, or supply such information, at the time or times required by law or regulations, shall, in addition to other penalties provided by law, be guilty of a misdemeanor and, upon conviction thereof, shall be fined not more than $25,000 ($100,000 in the case of a corporation), or imprisoned not more than one year, or both, together with the costs of prosecution. . . .
Your friend's employer will most likely report the salary/wages to the IRS so the IRS will know that your friend did not file tax return.
Second. Immigration. Assuming your friend does not have EAD, he might think that by not filing a tax return, he can somehow hide the fact that he is working without authorization. That may be true, the USCIS may or may not check tax records at a later time to determine whether somebody worked without authorization, but at least there will be a question on the application and your friend will have to lie. If caught, he will be denied the petition, at the least. Also, if the tax return is not filed, and the IRS institutes a proceeding of some sort, then your friend may have something on his record which he should also report to USCIS.
Now, the problem of unauthorized employment comes up. But it is a less of an evil than potential misdemeanor or outright lie to the USCIS.