the most common reasons an older person would be on Coumadin would be either atrial fibrillation or a history of DVT (deep vein thrombosis in the leg) or pulmonary embolus. Yes, there are some conditions where blood clotting abnormalities run in the family, but most of the time the clot or PE is due to some external cause (surgery, immobility, etc). And atrial fib - if you get that in the future you'll have to deal with it, but if you don't have it now there is no reason to base YOUR medical care on what your mother needs. So, unless you and this idiot PA know of a specific, inherited condition that your mother needs Coumadin, there is no reason I can see for you to not take Vitamin K, esp when you will NOT absorb it properly from foods and your level is going to tank unless you supplement. And then you won't be able to clot properly.
follow your levels. Take whatever supplements YOU need, not what your mother does or doesn't need.