Speak to your Manager and explain your predicament - if there is no conflict of interest (you mention that the work would be completely unrelated to the area you currently work in), and you have had a postive experience with your current company there should be no reason for them to veto the move. Also worth pointing out that having a well regarded alumni of your current company on client staff can do well for future business - tell your Manager you would of course promote them in new areas, talk them up etc.