Low visibility can mess a lot of people up - over here we get used to it and anything over 3-5m counts as good vis.
Personally I would work on buoyancy, trim and finning one stage at a time.
Start with buoyancy - trying to get to a point where she can comfortably hover at a set distance from the bottom. At this point it doesn't matter if trim is perfect as, so long as she can maintain a set depth, she is good.
Then add trim. Working out not just where weighting can/should go but adding in the fact that she can vary her trim by moving her arms and legs closer or further away from her body. Arms clasped together or folded in front and the legs bent up at the knees. Tipping forward= bring the arms closer to the body & lower the legs slightly (from the knee). Tipping backward= move the hands slightly away from the body and raise the legs.
Then finning. If her buoyancy and trim are good then adding non silting finning should be easier as her feet will be a lot higher than they were.
Another thing that might help is working on her fear of what she can/can't see. While doing the buoyancy/trim/finning, get her to close her eyes - initially for just a few seconds, then longer and longer as she gets more comfortable. She will then realise that her buoyancy and trim will be just as good even when she can't see.
Make a game of some of these things such as a competition between you as to who can hover best for longest, who can hover closest to the bottom (in a pool) without touching the bottom. If you push too hard, you could just succeed in turning her off to diving so everything has to be as much fun as possible.
