This strategy will not work if your client is balding and have a straggly scruff on their lip and face, the focus will always go to the hair that is thinning. Suggest to the client to go absolutely bald or attempt a buzz cut with a fully groomed, thick beard, sometimes you just have to take the plunge.
A stubble beard also goes perfectly with a balding head, and various lengths of stubble are determined by the strength and shape of the jawline.
If they have a weak or round jaw, then a full beard might be better, but if they have a defined jawline that they do not want to hide, then a stubble look would work better.
A word of warning to relay to the customer once you have shaved their head for the first time, they may find themselves feeling a little strange (not to mention slightly cold).