I had a vinegar mix leftover in my spray bottle, and scrubbing it out with hot water and dish soap just left the faintest hint of a soapy/vinegar smell. My plants have been enjoying their foliar sprays, so I don't think a small amount hurts. It's likely beneficial against pests.
I know that you use dish soap to treat fleas on pets - the soap allows the water to penetrate the fleas and eggs because of some science wizardry I'm too baked to remember correctly. It basically drowns them through their skin-equivalent, if I understood it right.
I wonder if half a drop in the spray bottle would do anything to help/hurt the stomata.
You should put vinegar in every meal and beverage of your brother's that you can get to for the foreseeable future.