I remember in high school, a few months after we first met, I was at my now-girlfriend then-close/best friend's house and recalled that she had told me she was amazing at Halo. Since we were talking on the couch with the tv on, I was like, "You know, you should show me your amazingness at Halo." She wrecked me the first two games, then I finally realized that she had given me a crappy controller with a response delay. After she gave me a good controller, I decided to destroy her for kicks.
Bad idea, never ever destroy a girl at a video game she knows well. She "play" punched (the punches increased in power) me for a solid five to ten minutes after. Another reason for her punching me could have been that I was crying with laughter and hugged her after I won; she hated hugs.
But seriously, never ever outright win against a girl in a video game she knows/likes or she will be upset, which may result in violence. If you win, make it look like you barely won. In terms of LoL, always encourage and praise her even if she doesn't do well. If she does very badly, teach her how to play better, but don't make it seem super important.