Google published a remote desktop plugin for the Chrome browser. It's not Open Source, but it is free (as in beer), and professionally written installation / setup instructions are available in multiple languages.
Actual remote access for you will be controlled by the user, they create a one-time passkey in Chrome and share that with you to connect to their system.
Here's the plugin page: https://chrome.google.com/webstore/detail/chrome-remote-desktop/gbchcmhmhahfdphkhkmpfmihenigjmpp
Here's the support page: https://support.google.com/chrome/answer/1649523?hl=en
For non-technical users adding a browser plugin is going to be much easier to understand than messing around with port forwarding and system permissions.