You've asked a tough question.
The page says the owner is looking to transfer to business to a new owner. I presume the owner wants the software to continue to be developed.
If there's an existing customer base, and existing revenue based on proprietary sales, then that's a monetary incentive for someone else to take over.
If the software is made open source, then part of that goes away. I have no idea what the market situation is for that product, but it comes down to asking what the benefits are.
Yes, there are moral ones (à la Stallman), but "open source" tends to focus on the software development advantages, eg, more potential contributors, more popularity leading to secondary income, etc.
In practice, these rarely occur. It's just that it does occur for the most popular and well-known packages.
So it seems much harder to find a developer willing to take on open-source development of a package like this, than one willing to keep it proprietary.
At best there might be an appeal to history, "if you aren't able to find anyone to take it on, then would you release the source in a version that lets someone in the future see it"?
That's tricky because it's an emotional appeal, at an emotional time, and you don't seem to have an existing connection to the author to build upon ("stumbling across this website" means you likely aren't even using the software), nor something tangible to offer.
Now, it's possible that someone here could be interested, but it seems not only unlikely (few discussions here concern retail sales of Windows desktop apps, or the underlying technology, and those that know those skills likely have existing careers), but doubly-unlikely as it seems there's little reason to think it's worth the effort even to consider the possibility.