I'm going to ditch a friend for my grandfathers birthday
Did you not want to see your friend?
I think I can get away with not hurting my friends' feelings if I just blame my grandfather for me having to ditch.
You should at least let your friend know before hand. You should not ditch your friends unless it is absolutely unavoidable.
I didn't even consider that. I think it'll be fine.
Using your grandfather here as a scape goat is pretty rude. You friend will understand and be happy you told them that you can't make it. Just say you forgot about your grandfather's birthday and need to miss your current plans with them.