Lenin

Sunlover,
It's a potassium sparing diuretic. It removes excess water and salt from the body.

IF it removes too much water from the body one can dehydrate: excessive thirst will occur and there will be no excess to urinate.

My standard is that if one doesn't urinate much and one is extremely thirsty, then cut back on the diuretic.
The same scenario happens a lot with my Lasix.

Also, if you're not peeing the fluid out then you aren't lowering you BP anymore.

Perhaps splitting the pill or skipping every second dose might be better for your mom.