you can move and attack with one hand, left or right.
NUMBER PAD, Turn off your number lock, it is arrow keys, and they DO work for movement, the difference is your down arrow, which you would only need on the aran for some moves (which can be hotkeyed anyway).
Simply set attack to Page up (it's moveable now), and now you can either use Page up, which is next to the number pad, OR page up, as the 9 key on the number pad, which is way more usefull.
Maple story can be played almost entirely on the numberpad, the exception being chat, and anything that uses the mouse.
My suggestion, is to turn off Num lock (its right there above 7 and next to /* and -), and then go to your windows > control panel, and set your mouse to left handed, and get used to it.
yeah, I do that, with a 360 controller, lol. but even then, even a 360 controller can be used 1-handed.
POV/Dpad for movement, and the left analog for skills and attacks, or abxy for movement, and right analog for skills fir the right hand.
if he gets JoytoKey, which is free, and has a wired 360 controller, he can do that as well, although I'm not sure which would be more uncomfortable, using a controller 1 handed, when your used to keyboard, or using the number pad, on the right, with your left hand.