Is it possible that my Green card can be revoked because of fake signature on the gren card lottery application?

Details:

I won a green card lottery in 2004 and moved to US where I presently reside.

The problem is that when I submite an application for green card I did NOT sign it. I did not know the rules because I did not speak English. My friend did it for me (US citizen). Now we broke up and he is threatening me that he will provide this information to USCIS.

Am I going to run into serious problem after he commits that action? Can my Green Card be revoked or something? What is my chances to win this case (I mean in court) if they will conduct legal signature analize / expertise?

Thanks.

Application was made online, no signature required.
If you already have an interviewed and you're granted a GC, you're fine.

Tell him if he doesn't stop, you will report this to local police.

I applied in 2003. Signature WAS required. I checked the official application guide for that year, it's stated there that application will be cancel if an applicant did not sign application personally. So, I'm concern about my Green Card. If they will start investigation it's quite possible that they will consider my application illegal. And I assume it can affect my PR Card. Am I wrong?