We consider non-coherent detection of M-ary FSK modulated signals transmitted over a slow, Rayleigh fading channel in a wireless relay network. The network consists of a single source-destination pair and a number of relays (L), which employ cooperative diversity. Performances of a counting rule and square law combiner are studied. We derive closed form expressions for probabilities of error for equal relay channel average SNR. For unequal relay channel SNRs, we resort to Monte Carlo simulations to estimate the error probabilities. We examine different combinations of M and L for a range of average SNR values. Although the square law combiner outperforms the counting rule for equal and small average SNRs, the loss in performance is not high. Simplicity of counting rule may be advantageous in these cases.