If Jacob dies during High Times the quest can still be completed for partial XP, but Julie Farkas will not start giving free medical supplies to the Courier afterwards. If both Jacob and Bill Ronte die during the quest, the Courier will gain infamy with the Followers upon turning it in. This can sometimes happen accidentally if caught in the crossfire with Freeside thugs.
"If I had two caps to scrape together I'd buy the materials to make my own, but my damn hands won't stop shaking. This guy named Dixon. Something about him seems sketchy, but I can't stop now. Last time I stopped, I felt like I was going to turn inside out. At this point, I'm mostly buying his stuff just so I don't feel sick. Speaking of which, do you have any spare caps?"