Yep already using WordPress HTTPS but in this case I was able to just comment out the call in the plugin file.
What's interesting is that since Open Sans is being loaded by WP core you do not need to call it again for the front end at all - it works without explicitly calling it in your theme. I wouldn't have thought that to be the case.
Thus any plugin would not need to call it at all unless they needed specific weights/styles not included by WP.