Shared editing of a flowchart for free

I use that app my self and was probably the one that gave it to you.

What is the issue?  You should be able to share with him and he can create his own from the file and share back with you.

It is just storing XML files where you like. I use dropbox myself.

Yes, but that still creates duplicates of the file. I currently don't see a way to send him a link and say, please edit this where he would have all the tools he needs online to do that. Please do correct me if I am wrong or send an example link that would allow me to edit a flowchart that was created by someone else.

You start by opening an existing diagram.  
There is an option for sharing under File->Share but for that you can't use dropbox and instead use Google Drive or OneDrive.  

Then you can also synchronize  https://www.diagrams.net/doc/faq/synchronize
When you collaborate on a diagram, synchronize the changes made by others to merge them into the diagram you are currently editing. The manual Synchronize feature works in the online version of diagrams.net with Github, GitLab, and Dropbox, and with the Desktop version on any files stored on any network drive.
If you collaborate on a diagram stored in Google Drive or OneDrive, external changes to your diagram are detected and merged automatically, unless you have disabled autosave or are working offline. In this case, you can also manually synchronize and merge the changes made by others.
Note: As there is no revision history in Dropbox, when changes are merged, the diagram file will be overwritten.

When you collaborate on a diagram, synchronize the changes made by others to merge them into the diagram you are currently editing. The manual Synchronize feature works in the online version of diagrams.net with Github, GitLab, and Dropbox, and with the Desktop version on any files stored on any network drive.

If you collaborate on a diagram stored in Google Drive or OneDrive, external changes to your diagram are detected and merged automatically, unless you have disabled autosave or are working offline. In this case, you can also manually synchronize and merge the changes made by others.

Note: As there is no revision history in Dropbox, when changes are merged, the diagram file will be overwritten.

Yes, but if you want to share the editing of an existing diagram by sending someone a link that they can immediately open and edit within the web browser and save, this does not currently seem possible.

Thank you, Scott!

It looks like the option for full automatic sync does exist after all and you can give it access to only one folder using Google Drive so that solution does work.  I will test with my coder and make certain.  Do you know if there is a way to have the text wrap automatically to fit within the box that I am typing in?

This is what I have so far https://app.diagrams.net/#G1lMFrIHCro5T7SI8LGWqhDLg08uAAmU-J.

That is what I was going to add. In OneDrive, creat a working folder for the one project and share just the folder.

In onedrive, it looks like you would have to share the entire thing but in Google Drive I can share just a folder, but that definitely works and allows me to create a link to the flowchart and allow people to edit.

Well, draw.io which turns into https://app.diagrams.net/ is what I like for that. You can save your drawing to dropbox or wherever you want and share the file (which is an xml file) with anybody to collaborate with.

Good luck!