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