The app simply replicates whatever is put into the responsive layout. Achieve it there and it should be in the app. Note that I say "should" as it's possible the conversion might not be able to handle certain operations.
I am not quite following what the client wants. From what you say the user has to tap (you tap in an app, not click) a word or an image to be taken to the TOC. Where is that word or image appearing in the first place? Are you wanting a page with words or images that takes the user to the TOC? That seems to be an extra step towards finding the content. In any case I am not aware of any way of creating links to the TOC, if that is what you want.
See www.grainge.org for RoboHelp and Authoring information
The course has some HTML pages with text and images. Nothing fancy. very basic stuff. The client wants some text to go to the TOC. Here is a sample text.
"Click here to view the ToC." -- The word "here" must be linked to ToC.
This is just one more way of finding the content.