Which memory upgrade gives a faster Mac Mini?
The difference in speed between matched/unmatched pairs will be completely negligible vs having more RAM.

Tell your friend to get as much as he can. 3 > 2 always :)

I haven't tested that particular case, but I have played with a Mac Pro (that offers eight memory slots) and although I have them in the "best possible scenario", when I didn't (for a while I was testing a different "less optimal" setup), I couldn't really notice the difference…

I recommend you head over to Crucial and fill the fields to find the type of memory you can install. They have a decent model/type database and tips to obtain the best combination.

But again, I recommend > memory rather than worrying about memory performance. More is better :)