Hi, I am new to the forum. I am not sure how to deal with a physical problem like double jointendness, a doctor would have a better idea how to correct that. However, a bad habit can be fixed. I have seen beginners lift their wrists to find the keys. If this habit is not broken from the beginning it is tough to get rid of later. Explain to the student how the line from the elbow through the tips of the fingers needs to be straight rather than arched at the wrist. Using a washable marker, draw a line from her thumbnail straight to the elbow (It will end at the inner elbow). Your student will then be able to actually see her wrist bend and help it relax.
Make sure your student is playing on the pads of her fingers rather than her fingertips.
If your student's fingers are extremely curved when playing, try to flatten the fingers a bit. The writs will naturally fall.
Are the elbows also moving out? Keep them from becoming chicken wings, If the elbows are kept at the side of the body, it is easier to lower the wrist.
Don't try to relearn music that has already been worked on. Using a piece that is below the students level so that notes and rhythms are not an issue, have the student learn the piece specifically with a low wrist. You can even exaggerate the wrist for a short period of time. As this will be the only goal of the piece, it may be easier for the student to achieve the intended goal. There are some nice collections of classical music from "real" composers for levels 2 and 3 that should work.
Of course, the dreaded Hannon and other standard technical studies, scales, etc. all can be utilized with a metronome at a slow speed for specifically targeting a lower wrist.
One more thing, (I am full of them) Take a picture or video tape the hands so the student can actually see what they are doing from the side. You may say, "Lower your wrist" and the student may think they are lowering their wrist, but from above, it's hard to see that motion. The student will benefit from a side view.