Some (most?) ftp clients support an editor for just that purpose. I know I have FileZilla set up to use notepad++ at work and editplus at home. All I do is run FileZilla, click on the website name already configured to connect to the site and the specific directory I want to 'land' in - and then click on the filename I want to edit. It appears in the editor, I edit, and when I close the file it asks me if I want to save it on the server. Why yes, yes I do. All done!
FileZilla keeps settings in a handy xml file so you can set it all up for her and then she really just points and clicks.
Failing that, a wiki that supports authentication - with your mom as the only authorized user - might be an option.