Stolen phone issue

My sisters iPhone was stolen. She had insurance so the phone has been replaced. Her issue is that the person who stole it is using in. He has signed into everything on her phone and it is still connected to her iCloud. So everytime he does something it messes with her phone. She has had to reenter her contacts 4 times. Her apps aren't working. How is he doing that even though she has a whole new phone? And how can she fix it?