Big River allows you to resend the email receipt for a transaction from the edit page for the transaction.  From the Transactions view, you can search for the transaction by running a search by name in the search bar:

Hover over the transaction you want to select and hit edit

Once you've completed that task, scroll down the page until you see the "Tasks" menu at the bottom of the page:

Hover over the tasks menu and choose "Resend Email". This will complete the task of sending an email to your donor if they didn't receive one.