The thing is, there's nothing constructive about flat-out telling someone they're bad. Tell her to stay under her turret to farm, or what areas aren't safe to be in. Flaming does nothing but distracts the team and ensures defeat. Some people have off-games, and I'd presume that she's within your level if she got placed in your team in the first place.
I lost a promotion series match where I was the jungler, and our top was 0/8 and our bottom was 0/7 and 0/8. Then, in the next game, I played support Lulu to a Vayne that DCed for 3 minutes. By the 30 min mark she only had 50 CS. It sucked, but I didn't complain about it.