Hi, Ted.
Editing the core plugin files is never the ideal way to go (for our plugin or any other), particularly for the reason you stated.
Instead, WordPress hooks (actions and filters) are the way to go.
With our plugins (and many others), we also have hooks unique to our code.
I believe two of our recent blog posts will come in very handy for you and your developer(s):
We also happen to have template override functionality, which is documented in our Themer’s Guide.
I hope this information helps you and your developer(s).
Please let me know if you have any follow-up questions on this topic.