How can a girl avoid being used?

My friend was in love with this guy. She was so happy with him that she even moved in with him. She got him expensive gifts and was looking forward to marrying him. Now after 1 year he wants her to move out. He says he met someone that is more adventurous in bed and hotter outside. My friend is devastated. She is feeling like a cheap hooker. How can I help her? Why men behave this way?

How can a girl avoid being used?
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