Only the person who controls or owns the Google Calendar can gain access to the iframe embed code. So, to set this up, the client will have to follow the instructions linked below to get us the iframe embed code.
You will need to have the page editing filter set to "Full HTML" to drop down into Source and paste in the iframe code.
Note that the Calendar must be made Public before you can embed it. When you check the "Public" box it says, "Making your calendar public will make all events visible to the world, including via Google search. Are you sure?"
