We have calculated the dipole moment of CO along the potential energy curve using the pure diffusion quantum Monte Carlo method. Good agreement with coupled cluster singles-doubles calculations has been obtained throughout. An accurate treatment at the equilibrium distance requires a multi-reference trial wave function for which we obtained a dipole moment in accordance with experiment. We discuss the Hellmann-Feynman theorem within the fixed-node approximation in the case of field-dependent smoothly varying nodes. It is shown that the Hellmann-Feynman theorem is applicable to this case due to vanishing boundary contributions. A comparison has been made between the finite field correlated sampling approach and a direct calculation of the expectation value for the dipole moment operator. In the present application both methods perform equally well.