Hi -
Thanks for the inquiry.
To answer your question, there are many instances in which a client can be a good selection for a recommender, particularly for a 2nd or 3rd LOR (if needed). It sounds as though, based on your email, that this person has been a "manager" to you and knows you well.
With that said, if you are not choosing your supervisor at the firm which technically employs you, then you may want to consider using the optional essay to discuss the reasons driving your decision.
I hope this helps.
Erin
Erin Schuhmacher
Senior Consultant
mbaMission
www.mbamission.com