Ash here from the Shopify team.
When running a dropshipping business, returns can seem a bit tricky to manage. Generally dropshipping suppliers don't accept returns unless there is some sort of manufacturing error, but that will vary between different suppliers, so you'll want to check with each that you choose to work with on their policies. For that reason, it's ultimately up to you how you wish to deal with these types of situations, as the buyer will be dealing directly with you, rather than your suppliers.
Before you decide how you personally want to handle returns, I suggest checking out these resources on dealing with returns here from Oberlo, and here from Dropified. These pages should be a great help in making an informed decision!