Getting a reference from a PhD student
There are a couple of major dangers in asking a graduate student for a letter:


A typical grad student has never actually read a letter of recommendation and has very little idea of how they are written or what the standard of comparison is.  It's easy to completely misjudge the style or miscalibrate the letter, in which case a letter that's intended to be helpful can turn out to be useless, or even counterproductive.  No inexperienced letter writer should ever send a letter without first running it by a mentor, so you would need to make sure the grad student plans to do that.
Graduate students automatically write from a position of somewhat less credibility, just because they lack experience.  Someone with more experience can compare a student to past students whose later career trajectories are known, but grad students generally can't do that.


One possibility is for the grad student to write a letter jointly with his/her advisor.  That may or may not be feasible, but it's worth checking.