Fdl tt suffering from necrosis, will it spread? Doctor says it will just drop off. Advice? (photos)

Your surgeon is correct. The necrosis will not spread. The wound will temporaily enlarge as the necrotic tissue is separated by the body from the body. Subsequently , the wound will contract and eventually close. Although, the quality of the eventual scar may be affected, sometimes the result will be difficult to differentiate from the rest of the scar that did not have this problem. 

I am sorry to hear about the problem you are experiencing.  Your plastic surgeon is correct: the area of necrosis will not necessarily spread. However, when the black eschar does eventually come off ( either on its own or surgically debrided), the area may "look worse before it looks better". In other words, once the black escort lifts off, you will likely have an open wound, requiring dressing changes.

 In the longer term, you may be pleasantly surprised how good the area looks. In the meantime, continue close followup with your plastic surgeon and play close attention to good nutrition etc.

 Best wishes.