Which is more likely to cause a person to devalue some other person?
According with the description of Mary, I think that she would devalue her best friend.

Explanation:
Elen and Mary have been friends for more time than John and Mary have been dating; this means that Mary could feel more betrayed because she trusted a lot in her friend, and respected her boyfriend. She's more likely to think that John does not like her because she's not enough good for him (because of her self-esteem), and that would generate more dependence (she would try to fix this problem because a part of her identity is at stake).
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