Localtunnel is a neat tool you can use to quickly share your local Flask server with a friend.
To install Localtunnel, open a terminal and run the following command:
sudo gem install localtunnel
Then, with Flask running at http://localhost:5000, open a new Terminal window and type
localtunnel 5000 Port 5000 is now publicly accessible from http://54xy.localtunnel.com ...
gem: command not found error? Download RubyGems
If you load the URL given in the localtunnel output in your browser, you should see your Flask app. It's actually being loaded from your own computer!
This snippet by Kevin Burke can be used freely for anything you like. Consider it public domain.