I told him he could run a Thunderbird, and that a Sempron would be underclocked - if he can't find a Duron or Thunderbird, running a Sempron on 133MHz FSB is alright if he just wants to browse and type emails, it'll probably be faster than a subGHz Duron anyway. If you're American Cowen, you may be surprised how difficult it is to find rare or old parts in some small countries!
But if his system takes Semprons, it'd probably be better with an Applebred 1.4 to 1.8GHz Duron or an Athlon XP Thoroughbred anyway, which I should've remembered to say.
Here is the lottery:http://www.stud.fernuni-hagen.de/q3998142/pcchips/
Compatiblity is important for newer chips like teh Applebred Duron and Sempron - the system won't even boot or stay stable otherwise. If you really can't figure out what motherboard you've got, an Athlon Thunderbird for 100MHz FSB is the best bet as it is sure to work.