Taking metformin all at once is not harmful although it may increase the chances of her having side-effects (particularly stomach pains, diarrhoea). There is a new formulation of metformin (Glucophage SR) which is designed to be taken once a day. The advice is to take it after the main meal of the day (usually dinner time).

Standard metformin is generally taken 2-3 times a day, with or after food. Your sister will not come to any harm and if she has no side effects and her DM is well controlled then there is probably no great need to change. Most people with type 2 DM are also on other treatments (blood pressure tablets, cholesterol tablets etc) and she should be encouraged to take her medication correctly.