I have the Omega 8004 and I haven't had a problem with Wheat grass. I grew about 5 trays and then got tired of dealing with the the mess. Plus wheat grass has a sickly sweet taste. It might be the way she was feeding the grass in that broke the screen.
The 8004 is the easiest machine to clean that I've tried. Very good for greens and ok for everything else. I have recently cut down on greens because they might have hormonal effects.