Sorry to hear about your dilemma. Your mum has done very well to achieve the milestone of owning a geriatric dog! Unfortunately, just like us, dogs develop medical conditions associated with ageing. Many of these problems are treatable but they often require a veterinarian to perform a thorough examination and further diagnostic testing such as blood and urine tests in this case. Your dear dog may be incontinent for several reasons such as a urinary tract infection, bladder stones, bladder sphincter incontinence or even dementia.
Urinary tract infections and bladder stones cause pain and discomfort and do require treatment. Bladder sphincter incontinence is often treatable with medications to help maintain the tone of the sphincter muscle. Dementia can also be assisted with some medications.
I would strongly recommend that you consult with a veterinarian to ensure that your mum’s dog is not in any pain. In the meantime you could purchase a dog nappy (there are many available on the internet) that may help with leakages around the house.
Best of luck