Why does my mother's urine smell so foul- she is 75 and I can smell from another room?

Incontinence. A very common reason is that women at this age may be leaking urine. She may also have a urinary tract infection.
Needs a urine test. Foul smelling urine is due to dehydration, infection, or from medications being excreted through the urine. She needs to be seen and have tests performed.
Possible infection. Urine can smell from a variety of reasons. Medications and foods can make urine smell funny or bad. We worry about infection and dehydration, however. She should definitely get examined.