Well, my character, Zafina is quite loyal to the Fire Nation, so seeing trespasser would be enough for her to actually initiate a fight with both Tachi and Mo. Or one of them. I already am imagining a flashy entrance right before the borders leading inside Fire Nation. It involves her dragon pet xD.
However fighting the avatar would be much more sufficient. Fighting a skilled fire/lightning bender as Zafina would definetely amp up the avatar to perhaps learn it even more. Perhaps the avatar loses, and believes that Zafina is Jiasi, unless you guys already know how she looks like. Once Zafina recalls the familiar face and see's that its the good avatar, she will let you both pass through. If not, she will definetely not, unless told otherwise by someone else. Upto you though. But Zafina is extremely skilled though. Since she's been training since she was four! Bending and other similar things like Close combat, or and martial arts.