After Alice joins up with Joe, the pair head over to the nearby town of Sienna in search of a pharmacy to for medical supplies. Eventually, once the two reach the pharmacy, they're met by a grouchy Isham, who tells them to leave. After discussing her diabetic situation, Alice persuades Isham to join up with them, and subsequently the group loot the building and a gun shop nearby. Satisfied with their findings, the group return to Joe's bunker and decide what to do from there.
If you talk to Isham as Joe, Then talk to Isham as Alice before Joe ends the dialogue, the quest will get bugged and the step to talking to the guy will not be completed still until the end of the game