Dear Phyllis,
I would say that the added vinegar would work just fine in milk substitutes 
to make non-dairy buttermilk.  The actual proportion I use is 1 tsp per 1/4 
cup of milk.  That would be 1 tablespoon + 1 teaspoon for a cup of milk, 
but I don't think that little bit would make that much difference...  In 
baked goods, does the "curdled texture" of buttermilk make that much 
difference?  The real attraction of buttermilk in non-yeast raised baked 
goods is to add the acid to help the baking soda and baking powder do their 
stuff.
I wish you and your friend well.
Ruth