She'd be better off throwing rocks. A half pound rock at 50 mph will give ~ 40 ft-lbs.
I believe the pistol she fired at the pavement near the feet of one of the yobs was a Walther CP88 CO2 pistol. They fire .177 calibre 5 grain pellets at 2.5 - 3 ftlbs. Hardly in the firearm range and its doubtful that you could even humanely kill a rat with that sort of power.
A 200 lb draw weight crossbow with a draw length of 15" will put out over 100 ftlbs, as will a 100 lb longbow with a draw length of 30". Although the British government has strict laws for licensing air rifles and air pistols over 12 and 6 ft lbs, respectively, (and making their owners apply for permission to own them from the Police), they do not have any such rules regarding bows - which are usually far more powerful. (But maybe I shouldn't say this on a public forum it may give the idiots at the Gun Control Network ideas as to what to campaign against when the Labour government has enacted all their hysterical anti-gun policy proposals).