How can you maximise the shell life of milk in milk bags, after having opened them?
Nothing at all. It doesn't matter how you close the milk, 3-5 days in the refrigerator is the time it will last in the refrigerator. 

If your grandmother wants to spend less money on milk, she will have to buy it in smaller packages. Even if they cost more per unit, she'll have to calculate the price when taking in account the waste from the bag.