It’s pretty easy to use, and does it all for you, once you tell it what you want (it shows you the keyboard and tells you how to choose your key changes).
I don’t know of any recommendations as to what should be used, but I suspect the Caps Lock key might be a good candidate (I have my Caps Lock set as a second Backspace, and my Caps Lock is on the Scroll Lock key). Might take a bit of getting used to though, and it would be a fair bit of use for the left little finger. You could add a second additional Space Bar on the Right Ctrl key, as another option (you can have as many as you like), or you could use both Ctrl keys as Space Bars and make the SpaceBar a Ctrl key.
On the other hand, my son, who types at 130 wpm, uses his index fingers to operate the space bar (he never had any formal typing lessons). Perhaps your client could try that. It seems to work.