You may need to be more specific, since there are lots of terms that could apply. Some of them are: proxy, representative, spokesperson.
talks poorly about someone.
That's hypocrisy, or being a hypocrite: "a person who acts in contradiction to his or her stated beliefs or feelings"
Find someone else, be on the lookout for other guys.
If you are involved with a person you catch flirting with someone else, you can confront them about it.
It shows how a person explains the behavior of someone else.
Such a person is a bully.
A bewrayer is a person who betrays someone else, or who divulges information to someone else.
A sympathetic person is someone who feels pity of someone else.
You should not date someone when they are with someone else because that means that the person you are with is cheating.
i say yu try listenin to sum Ms. Krazie songs she speaks the truth. :))
An educator is someone who teaches or learns from someone else.
imposter