When magnesium is in solution, it becomes ionic meaning it is not tightly bonded to a carrier ion. The product suggests it is the magnesium chloride form but does not STATE this clearly.
The fact that there is twice as much chloride in it suggests this.
Magnesium oxide does not dissolve much in water, so I don't think it is the form in this liquid.
Magnesium is ++ ( a plus 2 positive charge) so when bonded to chloride it would have 2 chlorides (chloride is - 1 )when dry. When dissolved it dissociates into the separate ions.
Ionic Fizz product is that way. Magnesium carbonate, dry and magnesium ion ++ in solution when mixed.
The only thing I can think of is that the hip pain, may be muscular and reflects a lack of calcium. Calcium and magnesium work together in the muscles, for contraction and relaxation. When one or the other is out of balance, then the muscles can act up.
With his osteopenia however, there is always a chance of losing critical bone mass in the hip bone, which takes alot of weight bearing and can ache when getting thin. So really I think he should report this to the doctor and perhaps get another scan or at least some X-rays. Some people develop hip bone necrosis, and this needs to be checked for.
There can be nerve pain in the hips and bursitis too. So a good check up would be a good idea.
This link shows the location of the bursae:
Tight clothing, like jeans may make hip pain worse or even create it. Tight belts too.