There is still the last AGPL version. The most effective retaliation is to make a fork that is much, much better than the closed version. Less drama, more work, I know.
Since it is free software, many user-friendly changes are possible that tend to conflict with commercial interests. (Indeed, someone already made a post about the author's privacy policy changes, making the commercial software less valuable with zero lines of code.)
While the author may make a few bucks, that kind of thing will not pass any due diligence, so his success may be quite limited.