//Unit1:
type
  A = Class
  protected
    procedure DoStuffA;
  end;

//Unit2:
type
  B = Class(A)
  private
    FOtherObject : TSomeOtherObject;
  public
    procedure DoStuffB;
  end;

implementation

procedure B.DoStuffB;
begin
  FOtherObject.InstanceOfB.DoStuffA;
end;

//Unit3
type
  C = class (A)
  private
    FOtherObject : TSomeOtherObject;
  public
    procedure DoStuffC;
  end;

implementation

procedure C.DoStuffC;
begin
  FOtherObject.InstanceOfB.DoStuffA;
end;

//Unit99
type
  TSomeOtherObject = Class
  public
    InstanceOfB : B;
  end;