Client needs a site to site VPN passing through Fortigate 100e

I have recently inherited a Fortigate 100e which I am not familiar with at all.  I have a client who has a router on the same site as the fortigate who wants to use the internet connection on the fortigate to create a redundant VPN from his edge device to his datacenter (he currently has a secondary internet link connected directly to his edge device as a primary link).  

So basically I have connected the client edge device to the fotigate directly and I have assigned a private lan segement to the link.  My issue is with applying a static NAT to the link so that I can assign a public IP to the outgoing policy.  I've gotten so far as creating the policy to allow the traffic to pass, but am unsure about creating a static nat for the rule.  I just need a 1-1 NAT since the clients traffic will be forwarded from his edge private lan IP.

Any help would be greatly appreciated.