Mac Mini 2018 which memory bank is which

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 :)

This memory will work (Mac Minis use SO-DIMMs, also called laptop memory), but it will not use the full potential of your motherboard. This pair of Corsair is great for a 2010 MacBooks with 320m Nvidia.

If you are able, you should seek and buy DDR3-1333Mhz SO-DIMM (also called PC3-10600).

ps. I have checked the prices and 200PEN (google tells me that this is around 75USD) is way higher than the price you should pay (40-50 USD).