There are three basic ways in which the eyes become too watery, and the causes vary based on the mechanism at play. An individual may experience watery eyes due to excessive tear production, inadequate drainage of a normal amount of tears, or inflammation in the eyes. Excessive tear production can actually be stimulated by overly dry eyes, superficial foreign bodies, cranial nerve V irritation, eye strain, and situational occurrences such as vomiting
and laughing. Poor clearance of tears can be caused by laxity of the eyelid or obstruction of the nasolacrimal (tear) duct that drains tears to the nose. Inflammation due to conjunctivitis, blepharitis (inflammation of the eyelid) or allergies, for example, can also lead to watery eyes. Constant, excessive watering of the eyes is a reason for your mother-in-law to consult her eye doctor
The first step in the evaluation will be to obtain a more detailed medical history and conduct a physical exam. Simple information such as the use of contacts, change in environment, presence of other allergies or allergic symptoms, and the presence/absence of any systemic symptoms can go a long way in helping to define the cause. Once this is established, her doctor will be able to devise a treatment plan targeted to the specific etiology than can help reduce the excessive water in her eyes.