So i was going to attempt to keep Rob alive during the red wedding. I saved when i got the letter. I was enlisted in Robs army for both of these attempts. First, i was actually with Rob in his army and got the note that he died. Then i was instantly taken prisoner even though i was in winterfell with rob... So i reloaded and this time took leave from Rob, and went to wait at the twins. I still got the note. But back in Winterfell Rob is still alive and well. I didnt have any battle or anything. But the note still reads that he was killed and i guess the game with progress as such?
EDIT: When i returned to Robb he said "Prepare to die scum" and we have a relation of 33... He was in the lords hall of Winterfell so there was no reply options and no battle... Also he is removed from the character list so i think i will let him die... seems buggy lol