You may want have the presenter install Display Menu app. This seems to be a common issue with screen sharing application with retina displays.
To avoid this problem and ensure Screen Share works properly when you are the presenter, we advise you to not use a HiDPI screen resolution. However the setting for this is unfortunately not easy to find on Mountain Lion. We therefore recommend you install and use the “Display Menu App” which is available for free from the Apple App Store.
Once you have installed the Display Menu app, you can click on the icon in the menu bar and select any of the non HiDPI screen resolutions. Screen sharing functionality will then work as normal on a Mac with a Retina Display. Please note that Macs without a Retina Display are not affected by this issue.