Tell the student that you don't mind taking him or her on (if that's the case) and will talk to the CFI about it. Make sure the student is aware that you will see the CFI about it. Emphasise the positive re you, the instructor, the CFI & the school are interested in the best welfare for all concerned.
Your organisation should have some form of grievance or conflict resolution procedure.
If possible try to find out more from the student & his or her instructor why there is this problem. Is it a recent thing? Is it a personality clash / instructional style / something else etc? Is one or the other experiencing unusual stress at home or work that has affected his or her behaviour or ability?
It may be possible to resolve the problem without changing instructors by discussing the matter with the CFI, instructor & student. It may be the student's expectations are unrealistic or some other circumstance that is not to do with the instructor etc
Alternativelyr all concerned may feel a change is the best solution.
A mature instructor should understand he or she may not be the best instructor for all students and that this does not necessarily reflect ability. Similarly the CFI should also be cognizent of this.
On a personal note I've been involved in this sort thing as CFI, preferred instructor and non-preferred instructor. I don't think it's wise to denigrate the student OR the instructor. The focus needs to be on the behaviour that is causing the problem.
Best wishes to all concerned.