Have the employee create another sheet in the workbook which will be used as a lookup table. The new sheet should contain the number in one column and the employee name in the other column. That sheet MUST be sorted by employee number at all times.
Now, in the regular sheet that she's working in, first make sure there is a column for typing the employee number into. Then, you can either replace the employee name with a VLOOKUP formula, or insert a new column to put the formula into so she can double check the results against the real employee name. Once you have it working properly, the column with the real employee name could be deleted.
When this works properly, she inputs the employee number, Excel goes to that extra sheet and finds the number then returns the employee name next to it, displaying it where the formula is sitting.