I'm not trying to argue that fast glass, lighting, and lotsof other stuff wouldn't make life a lot easier. But if the budget isn't there, it's not there. There are lots of situations where what she has won't work, but there are lots of situations where it will work. how about instead of telling her what she can't do and what she needs, why don't we tell her what she can do with what she has?
WIth that lens an a reflector. You can shoot all day long with available light. You're best off waiting until the sun get lower, like late afternoon. When the sun is a bit higher, try putting it behind and to the side of you subjects, so it rim lights their hair and shoulder, and use the reflector off to one side or the other, on a 45/45 degree agnle, to light up their faces. When the sun gets a little lower, try having it come at their face from the side, and put the reflector on the other side to get a cool cross lighting effect. If you were going to get anything, I would say that an SB-600/800/900 would be your best bet. You could use that for fill flash, or shoot it through the diffusing layer of your reflector, or bounce it off the reflector off camera, or shoot the flash straight up on the camera, and have you assistant hold the reflector over it on an angle to get a sort of glamour lighting thing. Lots of options. Practice is the best thing you can do at this point.