What to call someone that is currently studying for their PhD?
The title would likely be "Mr." / "Mrs." / "Ms.". There is no prepended academic title that means "will likely have a doctorate at some point".

In other news, a PhD student is probably not a good reference in the first place. You should look for somebody who has been in the game a bit longer and provide a reference that more plausibly compares you against a wider range of previous students.