Why do women tear down a woman their ex left them for?

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My boyfriend at the time broke up with his ex to start dating again and met me immediately after and she literally insulted my age (I was 26) and how he went for me cuz I’m “easier to manipulate”  we had a 12 year age difference but that’s not a big deal.
She told him “why would you spend this much money on someone you just wanted to sleep with?” 
Why do women tear down a woman their ex left them for?
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