He can also use 1:Many NAT,
With 1:many NAT, you can redirect traffic on a public port to any private IP address and port using port translation, and you aren’t restricted to using the MX’s public WAN interface (you can configure as many public IP addresses as your organization owns). This allows significantly greater flexibility for organizations who have one, two, or a handful of public IP addresses and that want to publicly host several services from different private servers, listening on unique internal ports.
Configuring 1:many NAT: map a single public IP listening on several ports to multiple internal servers.
I don't remember if you are able to ping It, but If you test a TCP port via telnet It should work.