It is highly recommended NOT to modify any core plugin code. When you will update this plugin you will loss all your modification.

Same result can be achieved using add_action( 'fep_message_table_column_content_avatar', 'your_function' );
Also if you want to remove own avatar you can use

if (($key = array_search(get_current_user_id(), $participants)) !== false) {
    unset($participants[$key]);
}

just after $participants = fep_get_participants( get_the_ID() ); line.

I am planing to add a filter to remove own avatar in next release.