What I would do is help myself to their patch, if it's worthwhile and backport it to the BSD-licensed version.
The code is my copyright, and the patch is a derived work of something of mine; therefore I own it and it falls under my license.
Not to mention that if you take someone's BSD program, and just slap a GPL on it, it doesn't actually hold. Once the users are aware that it's actually a re-labeled BSD program from another author, they can just go with that license.