Hi there, I haven't had a band, so I'm not sure about that part of your question, but I do know fluid retention can be a sign of underlying disease.
I had really bad fluid retention before I was sleeved. I have always had a tendency towards fluid retention and as I got older, and fatter, it got worse. By the end of the day my ankles and feet were like balloons, often my calves as well. It was incredibly uncomfortable.
I was tested for heart, lung and kidney problems. Thankfully in my case there was nothing serious wrong - nothing other than my obesity, I mean. I really think this is a symptom your mum should get checked out ASAP. It might be nothing, but she may need treatment for an underlying problem. Better to be safe than sorry...