Guidelines to become a guarantor

my sister has taken loan of rupees 2 Lacs from a non banking finance institute two yrs ago while taking the laon she has made our father sign as garantor and also taken a blank signed cheque for guarantor. Our father is 78 yrs old and not much are of documentations related to bank,She has false told our father that she is taking sig just to open and bank account and they need sign just for introduction. she has not paid the laon amount and now the outstandng is come upto 4 lacs, The bank has now put the gaurantor cheque which eventually bounced. I want to know in now how I can protect my father in case of legal cheque bounch case. also want to if finance company can accept 78 yrs old man with as gauarator. his income source is pension. how can we prove that the based on income and age my father is not eligible to stand as garantor in first place. Pls advice on how to get out of this case.