computer may have been hacked

What OS does the sender use?

Ubuntu is pretty secured from the Internet: applications such as Firefox and Thunderbird are unlikely tricked. And so are many other applications shipped with Ubuntu (as long as you install all the security updates, of course).

Security problems may occur whenever you install software packages from untrusted sources or when people other than you has physical access to your PC. To prevent the installation of malicious software packages, just be sure to use the official Ubuntu software sources.

Defending against attacks from people who has physical access to your PC is a complex thing. In general, you should use strong passwords and maybe encrypt your data. However this may not be enough, however other techniques are really complex to apply and to explain.

Finally, just a note about e-mails. Everyone is allowed to send e-mails on the behalf of someone else, or even with inexistent e-mail addresses. This is pretty easy to do and there's nothing you can do to prevent that. You can however attach a digital signature to your e-mails (a digital signature ensures you are really the sender).

You can read more about digital signatures (and also about data encryption) here: https://help.ubuntu.com/community/GnuPrivacyGuardHowto
You can also read more about general security in Ubuntu here: https://help.ubuntu.com/community/Security
Security is a pretty complex and long argument, so don't expect the pages to be easy. :-)

I hope to have given you enough information, but please feel free to ask if you need more help!

Get your friend to change her email password and it should stop.

All this means is that spher computer has a virus that's spamming people she has on her contact list. Most antivirus software for windoze will detect and remove this kind of virus. Changing password won't do anything

there is almost no easy way to hack and infect an ubuntu pc. viruses that are designed for windows won't run in ubuntu. just delete the malicious file. also install clamtk from software center and scan you pc once. it will detect and remove viruses designed for windows and linux too.