First, he could hire a lawyer to challenge the cancellation. USCIS will have to show him the basis for their decision. In other words, they will show him the letter. He can then chanllenge the letter was forged and ask USCIS to reinstatement his working permit (and nullify all the consequences of the cancellation).
If he found enough evidence that you forged the document, he could file civil lawsuits against you for all his lost salary and huge amount of punitive damage.
If USCIS found enought eveidence that you foreged the document, US federal government could file crimial charges agains you for mail fraud, lying to federal government and/or forgery. You will be look at serious prison terms.
Of course, if you never get caught, you would never face these charges. However, most criminals got caught thought they wouldn't get caught to begin with.