A breeders net / box will stress her out at this stage. And the fry are probably not the guppy's and you most likely have fry from another male at the pet store. I would bush together some plants into a corner and she may hide in there from the male and she can have her babies in the corner. If you see her birthing then i would plop her into a breeders net/ box. That way she wont be stressed at much.