From Google’s perspective, it’s a Facebook issue 
This is not a Wordpress-specific question, so someone from the Discourse team might be more qualified to respond on this, but given that this is an issue between two different services unconnected with Discourse (Facebook and Google), I suspect there is not alot Discourse can do. Keep in mind this issue affects many services using Google login when sharing content on Facebook. This is not specific to Discourse.
I’m curious, if this is a B2B case that requires you only use google login, where does the need arise to support a large number of users clicking through from Facebook? In most B2B contexts, users are not interacting with a forum through a Facebook share.
Moreover, the issue seems to arise from the fact that people are being auto-redirected to google login on clicking a link to your site. Another solution here is to let guests view the site without logging in, i.e. disable the auth immediately and / or the login required site settings.
If you’re sharing content on facebook, requiring login to view content is a somewhat odd move, on top of the B2B context?