If we ignore the possibillity of using a const_cast<> (Don't do it!), you'll have to make a copy of the string and assign that one instead of the original.
Code:
int validateXMLfile(char *xmlFile_name);

int Processor::InsertOrder(const string file)
	{
	int error = 0;

	char* TempString = new char[file.size() + 1];
	strcpy(TempString, file.c_str());

	error = validateXMLfile(TempString);

	delete[] TempString;
	
	if(error == 1)
		{
		cout << "\nERROR VALIDATING file: " << file << endl;
		return error;
		} 
	else
		{ 
		cout << "Order VALIDATED against schema OK "<< endl; 
		}

	}
It ain't pretty, but it works.