I don't know if this will help...but I had exact same problem with the Verizon FIOS router :/ I could still be in D2 for example but if I didn't use firefox or IE for like 2-3 hours...I'd have to reboot to get them to work again...
In MY case...it was the size of the small NAT table on the router....only 160 slots....which just being on D2 would fill up in about 2-3 hours....my solution was ditch the router....problem solved.
IF your friends problem is malware related then the fastest way to a cure is Malwarebytes Anti Malware found here: http://www.malwarebytes.org/
its a relatively new program and works fantastically !! They even have a very good support forum
Its free too!
tell your friend:
1. make sure she has the lastest firefox...its version 3.03 i believe
2. don't install toolbars on your browser..they are handy but slow things down and some are less than clean
3. check to see that something else like a running process or maybe another computer networked to hers isn't using all the bandwidth
4. DL Malwarebytes.....install it....update it and run a full scan
5. lastely there is also a firefox addon called NoScript found here: https://addons.mozilla.org/en-US/firefox/search?q=noscript&cat=all
which goes a long way to preventing problems with browser hijackers etc...