What is the ruling on one who takes a small amount of money from her grandmother without her knowledge to meet her needs, knowing that the grandmother is well-off and would not give her if she asked?

Theft is a major sin. The Hadith clarifies that when a thief steals, he is not a believer. A friend stealing from her grandmother is both theft and an act of disobedience (disrespect to parents/elders). She must repent and return what she took immediately, along with feeling remorse and resolving never to repeat it. A person's wealth and their refusal to give does not permit taking their money without their consent. Similarly, the small amount of what is stolen does not make theft permissible. The Prophet (peace and blessings be upon him) cursed the thief who steals an egg or a rope, resulting in his hand being cut off.