Maybe she's trying to get the right pH soil content? Some plants do better with a more acidic environment.
Hypothetically, she could be trying to help the plant, if in a misguided way. Urea, the primary component of urine, breaks down into ammonia, the most crucial soil nutrient/fertilizer for plants after digested by soil bacteria.
That said, you don't want to have that breakdown occur directly on the plant, as the bacteria will "burn" the plant. If you want to use "night soil", it's best to have a composting bin a bit away from your plants (and nose).
