How do I know which products have seen other customers when they took a look at the product that I am currently looking? I am developing a website with the Drupal Commerce module, and my customer wants me to implement a functionality to allow the purchaser to know which products have seen other purchasers/customers when they saw the same product he is currently watching. 

Do you know if any specific module for Drupal Commerce is available, or can I use a combination of modules that have nothing to do with the Drupal Commerce module?
Yes the feature you are asking after is this module.
Commerce Recommender

:: Updated, Here is a tutorial on how you can achieve what you are looking for 

Drupal Commerce recommender tutorial