BuddyPress.org

No, you didn’t understand what I was saying. Read again the first line on my post. You can use in your child theme only the template files inside bp-template. You can’t use the core files.

widget.php is not a template file. If you want to edit widget.php you have only two option.
1. Try to find a hook inside the file.
2. If you can’t find a hook then you can only hack the core.