BuddyPress.org

this falls into the activity-templatetags and the function bp_insert_activity_meta

you can filter on bp_activity_time_since to remove it

then just a matter of modifying your child theme file activity/entry.php and reference a new function to display the time-since where needed (since within the context of $activities_template loop – easy to reuse some of the code in bp_insert_activity_meta )