The editor is launched by git using bash scripting. So you should ensure your quoting is for the bash shell. You used single quotes but you have an embedded single quote in the directory name. You should probably also use forward slashes and make it unixy. Seethis answer for an example that uses Notepad++. In your case, I think the following should be ok:
git config --global core.editor "\"C:/Program Files (x86)/Programmer's Notepad/pn.exe\" -allowmulti -w"