If you are talking about using you DreamHost SMTP server, it is the same as your "incoming" IMAP/POP3 server: "mail.yourdomain.tld". If you are wanting to use your ISP's server, it will vary (and you have to get that information from them). Which one you choose to use is a matter of choice; you can use either.
If you are trying to set-up a client to use you DreamHost server via your friends connection, you may have to modify your port setup in your client if your friend's ISP has blocked port 25 (See the DH wiki article on setting up email clients).
If his provider (ISP) is different than yours, you may also have to go through other steps to access your ISP's server (some require specialized server addresses for "out of network" access, alternate client settings, etc - and some just don't allow it.)
AT&T in particular requires some machinations to use their server from when not connected to the net via their system.
Don't forget that, in a crunch, if you can't immediately get the settings for your email client sorted on a new connection, you can always send via "webmail", accessible at http://webmail.yourdomain.tld.