I was unsure about this too - really no medicine for cancer? They could replace whole parts of body, why not replaces the corrupted by cancer?
So why would he choose not to live?
And about the medicine - why is in the vault made universal medicine and not the exact medicine? If it is taken from the diseased molerats, could be used only for cure of that diesease and now it is not available, because the original molerats died and only the evaluated version was still alive? I guess it is there for me to have a reason to take it with me and if it would be cure for that disease only, there would be no reason for choice.
So lets say it is mainly for that disease and also for other similar diseases that you could get from mutated animals. On the other hand - are there really any diseases that my PC could have? So well, I don't know, maybe there is some other NPC with similar disease you can save, if it is not the father.