Recommendation letter from a PhD-to-be?
Typically reference letters should come from faculty, as you implied.  If you were supervised by a graduate student, ask the graduate student's faculty advisor to write the letter.  Very likely the faculty member will ask the graduate student for assistance in writing the letter.  This will get you the legitimacy and the content you need.  It can be awkward because faculty need to create the impression they wrote the letter themselves, but you can finesse it.  When I was a PhD student supervising undergraduates I just told them that I would ghostwrite the professor's letter.