Won't take off:

Wrong prop (try a different prop, in the recommended range for the engine)
Engine running poorly (Tune or repair engine to obtain peak performance)
Engine too small (Get larger engine or smaller plane)
Plane too heavy (Put plane on a diet, or fit a larger wing)
Wheels too tight. (Loosen wheels)
Wheels too small (Get larger wheels)
Grass too high. (Cut the grass, or find some pavement)
Mud/water too deep (wait until field dries, or find some pavement)
Negative AOA while on ground (adjust landing gear to gain level attitude, or slight positive AOA)
Wing not covered (cover wing and try it again)
Hold-downs attached (free the airplane from its restraints, and try it again)


If none of the above work, suggest that your friend get a boat or car/truck/buggy instead