Use events for this. You have access to the event manager in a controller and you can trigger an event that a comment is added. A listener can listen on this event and send an email, write a log, and more.
And if the channel for sending the notification changes in the future, just add a new listener. The triggers do not have to be changed.
It would be even better if you use the event system in your existing model. For example, when a comment is added to your model, you trigger an event.
Then you are decoupled from controllers and request handlers.
(I will try to add a code example later.)