Like in Facebook as soon any new comment is made, the activity automatically comes to top on the homepage of your friends. Here, we will be doing similar thing by making BuddyPress activity stream bump to top as soon any new comment reply is made.
Buddypress Bump Activity Streams To Top is a BuddyPress plugin by Gladwinput.com originally based on the OLD BuddyPress Activity Stream Bump to Top plugin which has been left unsupported. So what all you need to do is install, activate and forget Buddypress Bump Activity Streams To Top.