The work-energy theorem can be generalized to the motion of a particle along any curved path. The simple argument here is that the curved path can be considered a sum of many infinitesimal paths, each of which is a straight path. The force on the particle can be considered constant along any such infinitesimal path so that the work-energy theorem can be applied along it. So, it is also valid for the sum of these paths. The net work done is the integral of the work done along the infinitesimal paths, and the total change in kinetic energy is the sum of all the changes.
It should be noted that for motion along a curved path, the force may not necessarily be along the curve. That is, the force may have two components: one along the tangent of the curve and one perpendicular to the tangent. The force that does work on the particle and, as a result, changes its kinetic energy, is the tangential force. The line integral of the dot-product of the force and the infinitesimal line element gives the work.
