Should she leave him?

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I have a female friend who has been in love with a guy(Mr. A) for almost 9 years. She fell in love with him at a time when she was heartbroken. So her decision I guess was out of emotion rather than wise thinking. She then fell in love with another guy (Mr. B)who loves her very much and understands her. She says Mr. B keeps her happy and she experiences peace when she's in his arms. Mr. B makes her feel strong and special. Mr. A has taken her for granted. She says she would love to marry Mr. B but she doesn't want to hurt Mr. A and his family. What should she do? Should she leave Mr. A and marry Mr. B?