BuddyPress.org

I assume you’re using the Recent Comments widget. That is a WPMU widget and not specifically a BuddyPress Widget.

You have three options:

• Search the WordPress plugin repository to see if someone has created a plugin to do just that
• Search the forums on the WPMU site for an answer. If you don’t find one, then post a question.
• Hack WPMU’s widgets.php file yourself to provide the necessary functionality