This is a good intention, but unfortunately the way copyright law works, having the license be ill-defined means that nobody can really use it and be assured that they're above-board, particularly in something like an Apache or GNU open source project, or in a company environment.
Unfortunately, there's not really a good, widely used license that does what you want, mostly because you run into problems pretty quickly based on derivative works with what you want. Let's say somebody merges your email client with a browser - can they call that by a different name? Can they sell that?
Anyhow, I'd suggest looking at looking at the Apache or BSD licenses (if you want the broadest use) or the GPL (if you want to ensure that modifications to the code must be distributed with any binaries made from the code). Licenses are a bit of a pain, but you can just pick one of the common ones and it'll really help adoption.
* Redistributions of source code must retain the above copyright
notice, this list of conditions and the following disclaimer.
* Redistributions in binary form must reproduce the above copyright
notice, this list of conditions and the following disclaimer in the
documentation and/or other materials provided with the distribution.
Ok I think I read you, but lets say his new commercial product was proprietary; the product costs $100. If the license for his original Inbox2 product was GPL, he would be required to distribute the source code on his new product right? So my point is, if its a BSD license he can integrate that code into the $100 product and not have to share the source code. I'm no software license expert so if I'm mistaken please correct me here.
IANAL either, but I still think he'd be free to use his original code in a commercial product. By releasing under GPL, you give others a limited license to redistribute and modify the software subject to some conditions, but you keep all your rights, and you are free to distribute proprietary versions of the code.
Unfortunately, there's not really a good, widely used license that does what you want, mostly because you run into problems pretty quickly based on derivative works with what you want. Let's say somebody merges your email client with a browser - can they call that by a different name? Can they sell that?
Anyhow, I'd suggest looking at looking at the Apache or BSD licenses (if you want the broadest use) or the GPL (if you want to ensure that modifications to the code must be distributed with any binaries made from the code). Licenses are a bit of a pain, but you can just pick one of the common ones and it'll really help adoption.