They have this saying for a reason.
My BPD ex used to tell me the same thing. She was actually pretty convincing her in her argument too. Saying that most girls are catty, and backstabbing (reinforcing my already belief that most women are evil, and playing off the emotional hatred I have towards my mother). Needless to say, that relationship didn't just end bad, it was a disaster. Her having mostly male friends is just bad news. Even if you trust her, you can't trust these other guys. All guys are looking to get laid, they hold no loyalty grounds unless you are extremely tight with them (chances are, you don't know her "friends", or have only met them only a couple times). It opens opportunities to cheating, or if anything, opens arguments simply speculating on the fantasy of cheating. It's just bad news.
I don't want to diagnose this girl as a borderline, as I don't personally know her. But in my case with my ex, I found out that she despised females not only because of her mother, but also because she saw them as "competition". She needed constant supply of love, affection, and attention. If she had any female friends, they were extremely ugly. It takes the attention away from her. She was that insecure with herself.
You met this girl on an online dating site. Chances are, she's probably got a couple of screws loose. Harsh, but true. There's a reason she had to resort to the internet to meet men. There IS something wrong with her. You might just have identified it, and can save yourself the trouble before it even starts.
This girl is a pump and dump, if ANYTHING.
-Scars