You could write a simple proxy. Rather than having the client connect directly to the server on port 80, for example: SERVER (someserver:80) <----- CLIENT The client connects to the proxy, which forwards requests to the server: SERVER (someserver:80) <----- PROXY (anotherserver:8080) <----- CLIENT The proxy can record any traffic between the two.
So if you can't touch the client, and you can't even configure it differently, maybe you could do this: change the address / port of the server, and put your proxy on the original address of the server.
So, suppose the client connects to port 80 of the server: SERVER (someserver:80) <----- CLIENT
You change the address of the server and put your proxy in place: SERVER (anotherserver:8080) <----- PROXY (someserver:80) <----- CLIENT