Can a co signer be effected by bankrupcy?

I have a question for my dad. He is a 60 year old retired firefighter and has about 30,000 or more in debt. He is looking at options and considering to file bankrupcy, what I am wondering is this. His father co sighned a personal line of credit loan for the 30,000 two years ago, if my dad files bankrupcy, will his fathers credit get in trouble? Or will it effect my grandfather in any way? If it does then my dad shouldnt do it this way, we want to avoid hurting my grand fathers credit he has perfect credit..my dad does not, but my dad cant survive with this debt any more either..what can he do?? Thanks so much Sunni