I inherited a GPL project which I'm trying to improve. I know of an MIT library that could really help me improve this project. I know its developer personally and I read his blog, and he's made it clear on several occasions that he likes MIT and Apache licenses.
My questions:
The MIT license is GPL-compatible: http://en.wikipedia.org/wiki/MIT_License
According to gnu the Expat or MIT License is compatible with GPL.
If I interpret the two licenses correctly, the MIT license is the more permissive; you can do whatever you want with the code, including selling the software in original or modified form. This should include the right to distribute it in a bundle that requires the source to de available to end users. The only requirement from the MIT license is that the copyright notice and license terms are included in the distribution.
Compatibility between MIT and GPL depends on direction. As others have said, MIT allows GPL to use it, but I believe the opposite is not true. GPL does not allow MIT to use it. You are GPL trying to use MIT, so you should be in the clear.
What I don't understand however is your first question. Why is telling him or him knowing a concern? The MIT license itself allows, but of course it doesn't address the social circumstances you're hinting at in your question. I think purely ethically speaking, I wouldn't want to use anyone's code if I feel the owner wouldn't like that, especially if the owner is someone I know. I suspect the issue in this case is not a licensing one.
As mentioned by others you can use the MIT. FYI: Any code you mix or link against a GPL project must be treated as if it is under the GPL.