Appointments+: Sync Google Calendar w Office 365 Calendar

I set up Appointments+ on my client's site (http://www.vaderair.com/make-an-appointment) because he wants customers to be able to set appointments for HVAC services online. He uses the Microsoft Office 365 calendar and would like the appointments to show up on that calendar.

I think I can do that by syncing his Google Calendar to the Office one using IFTTT. I did some research and found a Zapier app that could work (https://zapier.com/zapbook/google-calendar/office-365/), but Zapier is not free. It doesn't seem as if this is an easy thing to set up inside Office 365 or from Google's side.

What I would like to know is what is the easiest way to sync the Google Calendar I'm using with Appointments+ (which is connected to the client's business gmail) with my client's Office 365 calendar so that he can manage everything using Office 365 and not have to even look at the Google Calendar.