Why does my friend throw herself at the guys I like even when she has a boyfriend?

The most recent case was when we were at a party. My friend claims to love her boyfriend. But we were talking about a guy I’m interested in who likes me back, and we met up with him at the party. I gave this guy my number, and then my “friend”, who had been encouraging me to go for him earlier, started flirting with him too and shamelessly gave him her number as well. When we got home, I asked what that was about, and she denied everything and looked embarrassed. This is not the first time she’s undercut me like this with a guy I like, why does she keep doing this to me after encouraging me to go after them? She has a boyfriend.

Why does my friend throw herself at the guys I like even when she has a boyfriend?
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