Have you ever messed up a exs new relationship just so you can be with them?

My friend messed up his ex girlfriend relationship by convicing her new boyfriend she was cheating. The new boyfriend dumped her and now my friend is dating her agin they seem happy but she doesn't known what he did. My friend says all is fair in love in war if she doesn't have a ring on her finger then he did nothing wrong. Do you known anyone who done anything like this before?
Have you ever messed up a exs new relationship just so you can be with them?
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