You can restart a workout plan that your client is following via the "Manage" link on the "Clients" tab of the Trainer Dashboard:

From the Manage Client view, click "Workouts" located on the left side of the page.

Click the "Manaage" link to restart your client's current workout schedule:

Adjust your client's schedule and 1RM info as needed, then make sure that the first workout of the plan is selected to the right of the "Choose first workout" header, which is the default choice 

Click the "Update Schedule" button, and your client will start the workout plan over from the beginning!