Your Mum sounds like she has a stricture. I had all of your Mother's symptoms and I was four months out from my surgery. It took my surgeon three months to diagnose me and in this time I drank 25mls of fluid daily and four strawberries. I nearly died because my potassium was so low my heart was going into failure. I pray for your dear Mum it is the most SHOCKING thing to go through but they did solve mine after the third month and five more surgeries later.
Go back to your Mum's surgeon and demand a endoscopy to see if it is a stricture (your stomach/pouch twists from scar tissue, staples, etc.).