If the student is having difficulty seeing the cell clearly, they could have too many layers of the onion on their slide. The cells are transparent, so it could be possible to see several layers of cells at once under the microscope. One thing they could do is to try to make a new slide or remove an onion layer or two.
Another thing he/she should try is focusing the lens. ALWAYS start with either the 4X or 10X objective. Once you have the object in focus, then switch to the next higher power objective.