It depends on how complicated your content/site structure is, authored by the user account that got deleted, but it can be fairly easy if it's just nodes and sum such content.
Use which ever migration method you're most comfortable with. Migrate module is popular, but Feeds may be more user-friendly? Don't hang on module reviews here ;)
One way to go about it is to create a separate dev instance of your site by copying the live Drupal folder, and restoring the database backup that contains your deleted user and content. Like a time machine viewing your site at a moment in the past :)
You can use Views to list all content from that user, export it as CSV, or some other parsing case. Views gives you a nice overview of what's there so you can check you got everything. You can output post times, statuses, references to other entities...
Then build Feeds importer for what you exported. Test the import on yet another dev instance of your site, the copy of the current db. TEST everything thoroughly before importing to a site that matters, this should be obvious. Migrations are notoriously tricky and long affairs.
You could go as far as to restate the user with account history etc with some more magic, but it seems you only need the content so it will probably easier to just assign authorship to another user or anonymous if it doesn't clash with permissions, or create a completely new user, you can name it the same as the old one but the ID will be different.