It's convenient indeed to send customer statements on the go, @dsember. What we can do for now is submit a feature request to our developers.
Please know that we're unable to transmit a statement using the QuickBooks mobile app. You may consider sharing this idea by following the steps below so they can consider this for future updates.
Go to the Gear icon.
Under the Profile column, select Feedback.
Share your suggestion, then click Next.
Visit our website or blog to track your requests and recent developments.
As a workaround, you can do this by going to https://qbo.intuit.com using your phone's browser. Once logged in, follow the instructions below:
Go to the Sales menu and click Customers.
Select the checkboxes for the customer you want to make statements for.
In the Batch actions drop-down, choose Create Statements. From there, pick the type of statement and fill in the fields.