your router/nat likely has some type of dcc message translation and its screwing up
get someone to add this line to alt-r/file/new
CTCP ^*:DCC SEND *:echo -st A DCC SEND WAS RECIEVED FROM $nick THE CONTENTS WAS $1-
they should get something like this in there status window
[12:00:00] A DCC SEND WAS RECIEVED FROM DaveC THE CONTENTS WAS DCC SEND filenamehere bignumber smallernumber filesize
* The bignumber is the IP of the sender and the smaller number is the port there using
If ones from you have bad junk in there, this can be caused by hardware intercepting the mesage and adjusting it.
Check the router settings.