We basically can read events from any source within Google Calendar, assuming you have access to the calendar (either because it's shared to an account you have access to in Google or because you have OAuth access to the account where the calendar originates) and then sync them over to any other calendar that you have access to. You can also assign some nice custom privacy settings to those synced events so that they show up with whatever level of visibility you want others to have when they view your calendar.
So in your GF's case, she'd sign up for Reclaim, set her primary calendar as the non-Treatwell calendar, and then set the Treatwell calendar (assuming it's a shared calendar that her Google login can access) as the source. Then events get synced and you're good to go!