This is somewhat difficult to answer without having more detail about the cause of the coma, the size of the stroke, the type of stroke, etc. However, in general, if an elderly person slips into a coma after a stroke, the prognosis is very poor. If they come out of their coma, the prognosis for a meaningful recovery (able to feed themselves, walk, talk, clothe themselves, etc.) is also very poor. Therefore, probably best to defer judgement to the doctors that are treating the person, but overall the prognosis sounds quite grim.