If you read the GPL, it is clearly designed to prevent EXACTLY this thing. I'm sure as it was being written, Microsoft was envisioned as the evil entity that would steal free software and mix it with closed proprietary code. The license is clearly and deliberately designed to prevent this.
The "other side" may not see it this way. But the "other side" must abide by all involved licenses, including the GPL. Their own license, and the GPL license. The GPL is clear that if you cannot abide by all licenses, then don't do this.
If you read the GPL, it is clearly designed to prevent EXACTLY this thing. I'm sure as it was being written, Microsoft was envisioned as the evil entity that would steal free software and mix it with closed proprietary code. The license is clearly and deliberately designed to prevent this.
The "other side" may not see it this way. But the "other side" must abide by all involved licenses, including the GPL. Their own license, and the GPL license. The GPL is clear that if you cannot abide by all licenses, then don't do this.