[northlandboy]

Yes, they can do that. They're planning on hiding their internal network behind their gateway's IP address. Since the gateway's IP address is a public one, it's not going to clash with your network.

This is perfectly valid, and will work as long as the connections are only from them to you.

Presumably at some point they will tell you what the IP is, as you'll need it to define the remote gateway. From your point of view, when you're configuring the VPN, the only object in what you define as their encryption domain will be their remote gateway.