You probably want to use something like DynDNS, there are many clients already written to update on connect if your friend uses dialup, whenever your address changes via DHCP if they have cable, or even just check every so often. I'm not sure if you can only make a subdomain of your domain dynamic, but I know they can manage your entire domain for you (for a small donation, IIRC) or you can use one of their domains like dyndns.org, dnsalias.org, etc for free.