Which is more likely to cause a person to devalue some other person? Scenario:
Mary and John are in a relationship.
Elen is Mary's best friend.

What will make Mary devalue Elen more?
Hearing that Elen likes John (romantically) or hearing that John likes Elen?

Let's assume that Mary and John have been dating for a few months. Mary is a dependent person with low self-esteem who would go to great lengths not to lose her boyfriend. All three of them are around 20 years old.
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).
Hope this help you.