Yes, people can change, but I think the person in question has to want the change and make it happen. No one else can do it, the desire to change has to come from within.
If I’d met my husband even a year earlier than I did, I wouldn’t have given him the time of day. His life was one big party with drugs, alcohol, and girls. He was every father’s worst nightmare and his own parents were seriously about to give up on him. They stopped supporting him financially. One day he took a look at himself: on probation for something really stupid, no education, job at a video store, shoes held together with string, and no better prospect in sight. He decided he did not want that life. Went home, quit the drugs, moderated the drinking, got a job, and went back to school. Now when I hear stories of the things he used to do, it’s like hearing stories about another person. He’s the most responsible, hard-working, ethical (but still crazy and fun) person I know. Those changes came only from him, not from his family, friends, or anyone else who thought he was destroying his life.
Edited to add: Whether or not to trust a person who claims to have changed is trickier. Only time will tell, but it’s always best to keep in mind that actions speak louder than words.