The horizontal forces are 1000sin 30 = 500 N and 300 cos30= 259 N. So the 1000sin30 force is more than 300 cos 30 N force.
The frictional force is always opposite to the motion force.
In the sum they have assumed the body is displaced upwards, and that the frictional force opposes it and so the body is stationary.
But on solving the sum, they get negative answer W= - 1917 Joule. This shows that the assumption was opposite. So they conclude that downward force on body is more, but still the body is resting because the friction opposes it .
Hence the frictional force acts upwards and not downwards, as previously assumed.