A block of mass m is placed gently into a long plank of mass m moving with a velocity $v_0$ on a smooth horizontal floor. If friction is present between M and m:

 

If we take the ‘block and plank’ as system no external force acts on the system. The velocity of C.M. will be constant always.

$v_c=\frac{m\times 0+M\times v_0}{(M+m)}=\frac{M{v}_0}{(M+m)}$

The direction of the friction on m is in forward direction and on M will be in backward direction. Hence work done by friction of the block will be negative

If we apply W.E. theorem from C.M. frame 

$W_{\text{friction}}=\Delta K={\left(\frac{1}{2}\mu v_{\text{rel}}^2\right)}_{\text{final}}-{\left(\frac{1}{2}\mu v_{\text{rel}}^2\right)}_{\text{initial}}=0-\frac{1}{2}\frac{Mm}{(M+m)}{(v_0-0)}^2$