Yes, by downloading the Invoice via the Breadwinner Invoice record. On the bottom of every Invoice Record in Salesforce you will find a link, "Download the Current Invoice".
This will produce the Xero-generated PDF of the Invoice you can save to your computer and then email. Because this PDF is generated by Xero, it look exactly the same as if you had generated it within Xero.
Save the PDF to your computer, and from there you can attach this PDF Invoice to an email.
Alternatively, you can (if you have a Xero Login) view the invoice in Xero and click "Email" which will send an email via Xero. This will also generate a URL of the Invoice for customers to visit.
When people ask this question, what they usually mean is "I'd like to click a button in Breadwinner and have that invoice emailed to the email address in Xero for that Customer. I'd like the Invoice to then be marked as Sent in Xero and also Breadwinner, and I'd like the customer to get a link or PDF of that Invoice with the email".
This is a totally reasonable and understandable request, and we wish we could do it. However, Xero does not expose this functionality through it's API. They might some day, but they currently don't. Once they do, we will happily add this feature to Breadwinner.
If you would like to encourage Xero to work on these features, you can vote for them in Xero's feature suggestion area.