She doesn't mention when it started paying her all her bills exactly but the cloud version started in 2013 and the CD from 2010 so presumably it's been her full time living for at least most of a decade. Even if it all comes apart tomorrow that's still a pretty good run for any single product/sole proprietor-ish small business.
Just wondering here, is there any way to protect oneself from that type of situation while at the same time keeping the source code open? Obviously, the name and logo are trademarked, but a third party could rebrand the program while using the same underlying source code.
The advanced features seem to be a good protection. Also the cost ist quite low, if you competing in the same market could you offer the service cheaper? I guess not. Moreover it is critical software for small businesses, I think many would pay for long term support on the original software...
Under the GPL, a competitor can certainly take the code, rebrand it, and sell it as their own but they are required to provide the full source code of whatever "borrowed" GPL code they distribute to the end user. This ensures that the source (and whatever changes/additions are made) cannot be taken and locked up by someone else, which is possible with more permissive licenses like the BSD and MIT licenses.
Open source == no protection. You rely on the morals of your customers. At any time somebody bigger than you can take your free source, modify it, make it better because they have already a base and sell it as closed proprietary software and you can't do anything about it.
This is not true for all open source licenses. Copyleft licenses such as the GPL and its variants prevent this.
Someone can distribute your software instead of you and thereby lock you out of any profits there, especially if they undercut your price (in the limit case they can distribute your paid software for free), but certain open source licenses prevent retroactive locking as proprietary software.
I don't see that happening. They would have to offer support for the new product and most of my marketing is word of mouth nowadays. Should work out just fine ;)