You can often find people releasing their software under permissive licenses such as MIT or BSD* complain that they can't use any GPL library and that it's unfair since their software remains open source.
Sure it's open source but their software is distributed under a license that grants permission to integrate it into proprietary software (as remarked by @airfishey). Wanting to include GPL software in it is self-contradictory if you wish to continue to follow the requirements of both authors:
MIT/BSD-licensed software author wish to grant permission to integrate their software into proprietary software; but they want to use a library under GPL...
...and GPL-licensed library author wants to prevent their software ever being integrated into proprietary software.
But if the will of the GPL-licensed software author is followed, then it defeats the purpose of the MIT/BSD license. In this case, what use would there be to keep the software under a permissive license?
In other words, why can't your friend just use GPL for their software as well?**
*I use generic terms such as GPL and MIT/BSD instead of giving version numbers, etc, because there is nothing specific to one particular version of a license here.
**My guess is because they have the (well spread) feeling that somehow the GPL license is wrong. There is nothing wrong with it. It used to be the standard, now MIT is more fashionable, but there is nothing wrong continuing to publish software under GPL.
For example, I prefer to use more permissive licenses as well but if I need to use a GPL library, I don't hesitate to switch to a GPL license.