Any jeopardy she might subject her plant to would hinge on the current media pH and how much vinegar she uses in her watering/fertigation solution. Will it 'cure' the chlorosis? It depends on what the cause of the yellowing is. Micronutrient deficiencies can occur in spite of the fact there is an ample supply of nutrients in the soil. High pH levels cause many of the micronutrients (like Fe [iron]) to combine with other elements and become insoluble and unavailable. Lowering the pH with vinegar, citric acid, sulfuric acid, phosphoric acid, nitric acid ...... can return these elements to an available state.

The best way to use white vinegar or citric acid is to start with a given volume of freshly drawn tap water. Add enough acidifier to lower the pH to 5.0-5.5. This might seem low, but the 'ideal' pH for containerized (I'm assuming) plants is about a full number lower than ideal for mineral soils, and the pH will rise as the dissolved CO2 in the water gasses off. Make note of how much acidifier it took, and add that amount to your given volume of water every time you water/fertigate.

There are a number of likely suspects OTHER than high pH that can cause chlorosis or blotchy yellow foliage. High soil temps, low fertility, over/under watering, a high level of salts in the soil, using a fertilizer too high in P, root bound conditions, mites or other insect predation, are just a few.

If she's reasonable, she won't kill her plant, but it's possible it may not have the desired result if high pH isn't causal.

Al

Thanks for all the great info! This should tell her the best thing to try first. Pet out.