My mother has the habit of ending the conversation by pressing the BACK button several times instead of hitting the red button. The perso she talk to can be heard after a few minutes (people are nosy and my mother is old). Is there an option or an application that contains a silent closing of conversation windows and was visible until the end of the conversation?
An alternative approach that might be easier for your mom: In Settings→Accessibility, there is an option to make the phone's power button end the call (here labeled "power button ends call"). That might be easier for her than remembering to tap the on-screen button.
Google's support site has an an article on the feature but it doesn't really say anything more.