The friend is there for security. End of. Be cool with it, if you say no it makes you sound like you might actually be a murderer or something!
I would consider asking if its ok for you to bring a friend? Make a quip saying your tired of girls hitting on you constantly and fighting of two girls at once gets tiresome

then to really through a spanning in the works have a female friend come, but don't tell them before.. And have her big you up

go out, entertain them all. Win the friend over, escalate with target.
But yeah, it's understandable for her to want to bring a friend if you met online. Good luck and close that ass