For everybody that has seen "I Am Legend"...?

So my friend and i were arguing over whether the lady who comes with her son [when she rescues him from the mutants at the last minute and stuff] is his daughter or not. He says it is because she has a tattoo of a erfly, and when his daughter was just a little girl they mentioned erflies at the beginning of the movie. But if you ask me, that just gives him a connection with her and he knows he can rely on her or whatever, like the erfly is a sign, but she can't be his daughter, because you saw his daughter and wife die in the helicopter. Basically, who's right?