let me guess: they are not ‘overdeveloped’, they are just big. I suspect that your client is female even though you did not say so. You would probably have a hard time getting her to work her calves even if you tried for fear that they get even bigger.
However, I would incorporate exercises that work the calves clandestinely. Calves are not just ornamental, they plan an important role in movement.
There is really nothing you or she can do about this. Genetics cannot be changed. You can have her do all types of exercises without focusing on the calves alone (for ex. no calf raises). Walking alone exercises the calf muscle, so even if you advise her not to do any weight training for her calves, they will still get worked out indirectly.
Genetics may play a role in the size of your client’s calves and there won’t be much you can do about that. However, you can still incorporate strength exercises without fear of “bulking up”.
A balanced strength training routine should still be part of your client’s program for many other health benefits–and your client may find that focusing on developing other areas builds confidence.