If you haven't seen The Walking Dead S5E5 yet, here's what happens:
At the end of the episode we see Jim finding his wife and kids dead, and the guy with the mullet (whose name escapes me) comes limping towards him shouting, "Jim! Jim!!" with a few walkers giving chase. Jim comes over and kills the walkers, and then Dr. Mullet says to him, "I need to get to DC. I can save the world." or whatever he says exactly.
So, if Jim knew Dr. Mullet well enough from the very beginning for the guy to know his name, how did he not know he was lying that whole time?