This is done with proxyquire.
I personally don't like the technique, but it's the best way that I've found to respect the "node way" and still be able to test easily. You'd do:
var proxyquire = require('proxyquire'),
dataModelMock = require('./mocks/dataModel');
proxyquire('path/to/helpers/dataHelper.js', { '../models/dataModel.js': dataModelMock });