Hi Frank,
It's a custom theme using Bones so Open Sans is not being loaded by the theme.
I've tried similar functions dequeueing 'open-sans' as well as the following plugins:
remove-open-sans-font-from-wp-core
Disable Google Fonts
etc.
None of them work and I still see the WP call to Google Fonts. I know it is from WP because it calls the 300 and 600 font weights.
I'm wondering if something might have changed in WP 4.1.+ or 4.2.2.
Anyway great plugin!