The first step would be to download the source from here:
https://github.com/stephenbez/memizr
Next you would try to follow the instructions from the file called
INSTALL.
If you don't currently have a Linux box to test this on, you can get
one cheaply at a place like
rackspacecloud.com.
The difficulty depends on what you want to modify. If you wanted to
change the color scheme and text that would be very simple. Adding
more features would be more difficult.
The code is licensed under the AGPL which I encourage you to read more
about here:
http://en.wikipedia.org/wiki/Affero_General_Public_License
I would see no problem with charging a membership fee to access the
flashcards on her website as long as there is a link to download the
complete source code for the application. Cards would not have to be
freely downloadable. Read the AGPL over carefully to make sure.
Steve