Lisanna could technically be considered Natsu's childhood friend, but I wouldn't consider that as her primary standing in the series. Characters have mentioned Lisanna's past relationship with Natsu, but we've never actually seen the story focus on this after her return. Whenever Lisanna is really involved with a fight or something significant to the main plot, her story is entangled with whatever is going on with Elfman or Mirajane. As far as her being a love rival goes... That pretty much went out the window during the Tenrou Island arc. Lisanna told Lucy to stick close to Natsu before the Hades fight, and Lucy did exactly that by hugging Natsu a bunch of times during that fight. If any love triangle is going to pop up at this point, it's going to be with Touka.
Diabolos is a guild full of dragon slayers though, and I struggle to see Lisanna beating any dragon slayer. Even Wendy. Maybe in a tag team with Mira and Elfman.Maybe she will get a chance against Diabolos provided those three aren't the weakest.

It's like the author way of saying he has zero idea on how to make her relevant to the main plot. So she's better off as Mirajane's shadow.