On day three, Dave contracts with Mary to trim her palm tree. George goes and trims the palm tree. When he is done, Mary's next door neighbor Sally comes over and asks if George will trim her palm. George agrees if she will pay him $100 in cash. Sally agrees and gives him the cash. George quickly climbs up in Sally's palm and begins to cut the fronds. George forgets to block the sidewalk that is near the palm. The first of the fronds falls and strikes Pam, cutting her face. George panics and tries to climb down quickly. George falls out of the palm and injuries his back. What is Dave's liability to Pam, George and Sally and why?