Cisco Router 881 Connected to Fortigate 100D

Hi Everyone,

I am new in a fortigate environment,  I have a fortigate 100D, one of our clients is going to connect their Cisco Router 881 as their VPN Router to our Fortigate Firewall, I gave them Private IP for their interface which is 172.16.2.2  and I assigned one port of our firewall also to 172.16.2.1 , i have 1 public IP that could NAT/ Routed to their CISCO Router 881, how can i Configure it our firewall so that they will be able to access their Router outside?

Please help me. Thank you.