At least an smart DM, those that puts rule 0 to use, could make the npc heal the bonus hp (1d6+CHA) because, most of the time, the npc would have a few hp. The other way is, that an npc is allied with the pcs, if the dmthinks it is an important character, it should be more stated out, in that case be buit more like a pc, and the problem for that case is over.