Even if your friend intends to buy a concertina at a garage sale, he or she would do well to truck on down to Boaz (which I've also heard is good, but I haven't ever been there) and physically try out the various systems. They're *so* different from each other! Not only in terms of capabilities, but also in terms of how your brain relates to the system. For example, I couldn't get the hang of the supposedly easy-to-play Anglo at all, but I love both my arcane duets!

Here's more about duets, just in case you need it:

I have both a McCann and a Crane. They're called duets because, theoretically, since the low notes are on the left and the high notes are on the right, you can play duets with yourself.

My McCann system Wheatstone, circa 1916, uses all four fingers on the right hand (first finger, third finger, second finger, fourth finger and so on up the scale), optimized for the keys of C and G but fully functional in other keys if you're willing to do some mighty quare fingering. The 1-3-2-4 pattern makes it fast for the right hand, but I find the (different) fingering pattern for the left hand almost completely incomprehensible. So I can play whatever I want, but pretty much only with one hand. That's why I bought...

...my Crane (Lachenal, circa 1925), which uses three fingers (first finger, second finger, third finger and so on up the scale on the right land; third, second, first, etc., on the left). I could actually play with both hands the first day I got it! Hooray -- but the downside is I find the 1-2-3 fingering makes it very slow. OK for Morris and sea songs but doubt if I could manage an Irish tune, for example.

Good luck