FortiGate Internal NAT

Hello everyone,   I would like to create an IPSec tunnel with one of our customers. Unfortunately, the customer already has our private subnet 10.150.0.0/24! Now the question is what do I have to configure VIP, SNAT, ..?  (I'm overwhelmed) The customer is unable to NAT the subnet. Could you support me on this. My Gateway: FortiGate 501E Thanks