You cannot just "open" a database backup. You have to restore the database. For this, you first need to install PostgreSQL. The latest version should be capable of taking backups from older versions, too. But older version may have problems with newer features in the backup. So your version should be at least as recent as the source.
How you install PostgreSQL depends on your OS that you cunningly kept a secret. In Debian it's basically a matter of:
apt-get install postgresql-8.4
(Or you take postgresql-9.1 from the backports.)
Depending on the format (there are various) you can then restore the backup. If the backup holds a complete database cluster (produced with pg_dumpall - ask your friend!), you can simply:
psql postgres -f /path/tp/file.sql
To view the data you would best use a GUI like pgAdmin. Or you learn how to use SQL. :)
For more advise you would need to provide more information. Like a short(!) sample of the backup and how it was produced.
For more about PostgreSQL start by reading the manual or one of the thousand pages on the web.
But really, your friend should just export the relevant data do a CSV file that you can open in Open Office Calc or some other spreadsheet application. Or even any text editor.
Best Answer
PostgreSQL database dumps are normally produced by the command line tool
pg_dump, and expected to be used via eitherpg_restoreor simplypsqlcommand line tools.However, you can both backup and restore from PgAdminIII. The backup option is available via right-clicking either the server or any database or schema. The Restore option is available via right-clicking a database or schema.
Here's a step-by-step example of how dump or restore a database using PgAdmin.