A senior for whom I do some volunteer work has a leaky (thin stream) Moen single handle faucet which is making his water bill go through the roof. The handle is quite loose and if you move it around you can find a sweet spot where the leak reduces to a constant drip (as seen in pic). He is on a very fixed income and the luxury of calling a plumber would mean having to make do without some other essentials.

My question is; how to best determine the cause of the leak in order to try and repair it? I should add that the connections underneath are very inaccessible so I'm trying to avoid having to change the whole thing - that, and the not wanting to be on the hook should anything get damaged.

I will admit that while I've changed the kitchen and bathroom faucet at home, I wouldn't call myself a really handy guy so I'm looking for the easiest yet best fix, which I know isn't asking for much.

Thanks in advance for any and all advice and suggestions.