What do you want the EdgeRouter to do in this case? Firewalling? Or? If there's nothing fancy being done on the Fortinet, you could replace it with a dumb switch...
Assuming you do want to do something smart, proxy arp mode might be a way to do it? EdgeRouters are L3 devices, so they can't do much as a L2 firewall if that's what you were after, but with proxy arp and some creative routes you might be able to achieve it.
