If you have introduced him to shankar for the sole reason of forgery than you will booked for criminal conspiracy.

If you have not benefited/shared in the amount and have no connection whatsoever in the act, neither you introduced him for this act and after introduction, they themselves conspire to commit this forgery and cheating,  than no problem to you.

Dear Sir,

Nothing to worry Sir. If have not received any amount as commission or otherwise you will not be trouble. 

Don't worry. Nothing will happen. Don't go into too much of technicalities. if you help them in generatiing fake invoice then you are liable 

And you were informed that this all is fake and still you done. Definitely you are partner in crime. How much is the actual committed amount ?

The possibility of your name being revealed at time of investigation cannot be ruled out

Had the matter been restricted to only introducing your friend, that would not make you liable 

However since you have generated invoice for the company which deals in fake invoicing to claim ITC benefit, even the accountant becomes liable

But you can always take defense that the invoice was generated on instructions of company and you had no knowledge that it was a matter of fake invoicing. You can say that in the normal course you make several invoices for your client's business transactions, but which one is legitimate and which one is fake, cannot be within your knowledge 

You have no direct involvement in the dealings of your client and its business associates 

 any person who does the following will have to pay a penalty extending upto Rs. 25,000

• Helps any person to commit fraud under GST
• Acquires/receives any goods/services with full knowledge that it is in violation of GST rules
• Fails to appear before the tax authority on receiving a summons
• Fails to issue an invoice according to GST rules
• Fails to account/vouch any invoice appearing in the books

penalty would be imposed upon you as you have helped fraudsters in committing fraud 

• Every taxable person, on whom the penalty is imposed, will be served with a show cause notice first and will have a reasonable opportunity of being heard.
• The tax authority will give an explanation regarding the reason for penalty and the nature of offence
• When any person who voluntarily discloses a breach of law, the tax authority may use this fact to reduce the penalty

Encouraging or assisting a crime is itself a crime but you can defend engage a expert senior lawyer. 

There was no actual invoice and bills , all were forged by you. To your horror, even you are involved actively in crime than.

See in case if in the statement if any of the accused reveal the name and the officer start inquiry  against you  then there can be problems. 

See you can be implicated in the inquiry. 

See if these transactions are opened before the court then in that case you can be implicated  in case you receive notice then in that case get anticipatory bail from the court to avoid the arrest.

You can report the same by keeping your name discreet. 

If you are not involved in the said fraud or scam, why do you worry about it?

You make sure that you are not involved in this racket and be clear about it especially when somebody makes a mention about you also being an associate of the fraudster gang, you may obtain AB at that time and may file a petition to quash the charges agaisnt you based on your innocence and documentary evidences to your support.

Knowing it as a fraudulent transaction, you have done it intentionally, hence you may be implicated in the case if  any to be filed by the authorities concerned.

So be watchful and file an application seeking 
AB immediately once you have been notified about it. 

Yes problem may arise for you but you can defend yourself by giving the statement that you don't have any knowledge about the fraud done by them and you just introduced nagraj to shankar.

You should just deny of taking any money from them and generation of any bill if done through your ID