Have them look at, and consider just adopting the MS-PL?

https://opensource.org/license/ms-pl-html

Microsoft used it a ton, until they eventually just made everything open-source fall under the MIT license.

Some people will still be angry about it (I got a downvote for just mentioning it elsewhere on this thread) but as the person who built your software, you have every right to license your software as you deem necessary. There is a cost to what you've built and you have no true obligation to give everything for free.

On that note, as far as I can remember the MS-PL is OSI approved already.