One of the hardest things when you work for yourself is delegating tasks to others.  

So if you've got an assistant who helps you out, then that's amazing news.  It's a big step forward.  

But what about if your assistant is helping you with client work?

How much do you charge your client?

In my view this is completely the wrong way to think about it.  

Your client isn't buying your time, they are buying an outcome.  

The fact that you delivered that outcome, or your assistant helped you to deliver that outcome is immaterial.  

Your rate is your rate and all the client need care about is if you get the job done.