I have a customer on my server who has request
that I point a subdomain DNS level to a IP address that is not associated to my server.

Is this possible or are all subdomains tied to the same IP as the parent domain?

the short answer is that a sub domain can have a differnet IP to the domain

OK, so what do I need to do for this customer?

Do I go into the DNS and change the IP address for that subdomain?
Do I need to also register it with EV1?

What do you mean register it with ev1?
As I understand the ip is on a different machine
so it does not involve ev1

just set the A record for the subdomain to the ip should do it

Good point!

I changed the A record and it seemed to have worked.

Now what would be the purpose of doing this.
Is it mirroring a site or redirecting it?

are you must be talking about a cpanel thing.

I don't use the ev1 dns I prefer to have dns seperate from the data center where I'm hosting.

The only time I need to deal with ev1 re ip addresses is when I want to setup ip for a ip based site for use with apache mod_ssl
(they are always very quick about it too)

anyway glad to help