If I am understanding you correctly, that means you want your AJAX
request to return HTML markup instead of data, right? If so, you
should really use a view component like a partial, which can itself
use helpers just fine. In the second edition of Agile Web Development
With Rails, Chapter 9 ("Task D: Add a Dash of AJAX") has an example
of this technique.
Controllers should be about executing business logic and providing
data to the view, not being the view themselves. Helpers are *view*
helpers not *controller* helpers.