Sounds like she's getting "lost" 'cause you end up swimming away from her. So..... why don't *you* try swimming next to *her*? Since you're such an awesome diver, you should be able to slow down to her pace and stop if she stops. That way the only way she can get behind you is if she swims backwards
I can relate.... she is uncomfortable, her buddy swims away from her in low viz, she gets more uncomfortable as she is now alone and "lost" so she follows her training and surfaces!
I gotta say that if she is willing to dive the low viz, you may need to be willing to support her in getting the training of which boomX speaks.... Proper preparation and a supportive buddy go a LONG way toward increasing comfort in low-viz cold water.