How much coffee is too much for my mom?

All my Mom does all day is drink black coffee (with caffeine). I know she drinks about 10 cups a day. She acts nervous all the time.

I mentioned this to her doctor and he said caffeine was good for her brain. but isn't there a limit if she is drinking too caffeine and no water? She's done this for a long time, so I bet she's kind of used to it by now. But hate seeing her act nervous.