Hi Paul.
Thinking outside the box- can your mum get a milkman to deliver bottles of milk (they do still exist in some areas!)?
My mum still has a delivery, alternate days- and milk bottle tops are easier to open than plastic bottles.
Her milkman also delivers bread, eggs, potatoes etc when required, which is a great help particularly in the bad weather.

My dad had only one arm and we never thought of him as disabled when we were growing up- he could do virtually everything, finding other ways of doing things. What about your mum sitting down and gripping the bottle/jar between her knees?

Hope she heals well.