It doesn't matter if you do it in the helper file or in the main file. The helper file is just a nice place to stuff away this kind of code :)
To call the model you need to include it first since it's not autoloaded. That means usually using something like require_once('components/com_content/models/article')
.
After that you can instantiate the class with $class = new ContentModelArticle
and call the method $item = $class->getItem()
.
That should at least work in theory :)