It's in Lulu's first CG scene; where Tidus observes through the tent curtain Lulu and Wakka talking. She say's that line, and then when she walks away, she says, "Enough, Wakka."
That's where I think it is; I can't picture it taking place anywhere else.
Although, now that I'm looking at the scene, she doesn't say what you've quotes.
Here's how that conversation went:
Lulu: He does look a lot like Chappu. I was surprised too, the first time I saw him. But no matter what he looks like, he isn't Chappu. You shouldn't have brought him here in the first place!
Wakka: Yeah, but...He needed our help.
Lulu: Excuses again?
Wakka: Yeah, but...
Lulu: That's it. No more. Enough, Wakka!
Maybe that's what you were looking for? Sorry I can't help further. :/ I can't remember another time she and Wakka are alone and she gets angry about Chappu.