Despite her protestations of innocence, your friend has almost certainly got a virus or some sort of spyware that is catching all the email addresses she uses.
The only other possibility (apart from simple co-incidence (*)) is that her ISP's servers are infected, which is highly unlikely.
Antivirus software won't necessarily get find or rid of spyware, since it isn't strictly a virus, so get her to install some anti-spyware programs as well, and see what they find. It's a pretty good bet that it'll turn up something nasty -- and probably more than one.
(*) if you want to disprove the simple co-incidence theory, just set up a few extra email accounts, and get your friend to mail them all. The more spam you see after that, the less likely it is to be co-incidence.
Hope that helps.
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ps -- it probably doesn't make you feel any better, but I'd give my back teeth to have that little spam in a day! I run a couple of big web sites, and if I didn't have some aggressive filters in place, I'd be getting several hundred spams per day. Even with the filters, I still see a couple of dozen every day.
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