Low Vitamin D levels

I wanted to get your guys opinion on vitamin d deficiency
My friend got these results:

Vitamin D (25-OH) (tested)

Calcium 9.29
Vitamin D nmol/L 37.5
magnesium wasnt checked... (why?)

Should a low level of Vitamin d indicate a low levels of vitamin d's co factors?
His doctor subscribed him with D3 IU200 (in drops form x10 times a day)

FireFist - Vitamin D should be between or 125–200 nmol/L or 50–80 ng/mL

It's rare to be checked for Magnesium levels, and also blood levels are not that useful anyway.

Magnesium is still useful even if blood levels are "normal." The fact is, serum tests do not reveal how active a mineral is in the cell, and that is what matters most.

Your friend probably needs to take more like 10,000 IU for a few months and then re-test to see how their level is.