Girl's Behavior

Should I tell the girl I'm talking to that her best friend is coming onto me as well?

I started talking to this girl I really like. We are getting to know each other. Her best friend, who I've never met (I know they're best friends because I've seen her in this girl's photos together) has since followed me on social media as well and inboxed me with flirty messages, even though I can see she already has a boyfriend from her profile. I'm flattered by the attention from her friend, but it seems she's not a real friend to the girl I like. I don't think she knows her friend is talking to me.

Should I tell the girl I'm talking to, or would it seem like I'm trying to stir the pot between the two friends?

Updates
+1 y
Details: Her friend messaged me asking for a threesome.
Should I tell the girl I'm talking to that her best friend is coming onto me as well?
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