I don't know of a reference off-hand but here's one way to think about it. First, one can think of as , the set of homotopy classes of maps. Notice that a cellular model for can be taken to be union higher cells that kill off the higher homotopy groups. Second, any map can be replaced by an inclusion , where is the mapping cylinder and it has the same homotopy type as that of . This works in the equivariant setting also. The third fact is that if any equivariant map induces an isomorphism in cellular cohomology and acts freely on both and then induces an isomorphism on equivariant cohomology as well. The equivariant cohomology can be thought of as maps from spaces to up to equivariant homotopy.
Now think of as in inclusion and there is a long exact sequence in cohomology
which tells you in your case that if . The kernel of is just the image of in . Thinking of as relative homotopy classes of maps from to . These maps only probe the skeleton of because if . And since any map can be made homotopic to a cellular map we need only study homotopy classes of maps from the -skeleton of to the -skeleton of which is . These are precisely the different ways of factoring a given equivariant map via (all upto equivariant homotopy).
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