Has your mother got any other conditions, such as heart problems?
My father had a heart problem which caused swelling in his ankles but which was largely under control with medication. However, just like your mother, one night he found that his legs were 'leaking'. He called the surgery's out-of-hours number and the duty doctor deemed the problem serious enough to require an immediate home visit, which then resulted in my father being admitted to hospital very shortly afterwards.
So, in certain circumstances, 'leaking legs' can be an indication of a medical emergency. However, as Mamya's link indicates, lymphoedema itself isn't that uncommon, although it would be important for a GP to work out what the underlying cause might be. (My own lymphoedema is due to cancer in my lymph glands but there are lots of other possible causes).
Phone 111 to seek advice.