As far as I know, the issue is that Fortinet hasn't opened up their API to the level of other vendors. I haven't done that integration myself, but from 'having heard' there might be a partial integration via sending RADIUS accounting to your firewall. That doesn't include role and device information, but the IP-username mapping may work. As said, I have not tried, nor I have specific documentation on how to set that up on the firewall side. I assume that configuring RADIUS accounting on the Aruba side is trivial for you.