Cheque and stamp paper given as guarantee what action can lender take

Hi,m a 28 yr old female.My brother borrowed 2.5L from his friend asking me to be the guarantor.I have given an a/c payee cheque in lender's name (signed but amount and date not written)and blank 100 Rs stamp paper(signed by me) as a security.But they both didnt confirmed me when the actual transaction happend.10 Days after i gave the documents,i got to know that my bro ran away sumwhere without informing.till date we cant locate him neither have any contact with him.His friend is now asking me to refund the amount on his behalf otherwise he will take legal action against me.Please help what to