Is it possible to put a condition on a validation rule that will stop it from triggering for particular users?

We need a field to be uneditable (unless blank) for all but one user (our marketing automation platform Eloqua). 

public class A{
  public class B {
  }
  public class C {
       public A.B show(){}
  }
}

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Look above exampe. I have one inner class C.I need to create instance of that class.
Can any one help me to create instance of class C and call method show of class C.