You can say "i am not sure about that - I'm not a plumber and can't tell where the leak is coming from, nor if I could fix it.' It is okay to say 'sorry, that is beyond my abilities. She is probably thinking - he is such a nice man, and, tbh, plumbers are quite expensive and many on a fixed income really can't afford it.
Any help that you are p$&s about giving isn't really helpful - as it causes you stress (which you can't afford with your high blood pressure), and resentment which leads to poor relations with neighbors.
Neighbors can be helpful and neighborly, and still set boundaries.
I think you can very nicely say that you would love to help but you don't think you can because it needs a real plumber and you wouldn't know how to do it and might make it worse!
