First thing to check are the BT connections to first ensure that broadband is activated. no extension leads are being used and that all outlets have a converter fitted and the rem has not been exceded.
second is has this system been set up for this, its presumed he is useing USB has this been enabled,( not always a common setting on win 98 as many did not have USB from new ) have all the drivers been installed and has the broadband been set as default connection.
Wanadoo normally provide a good set of easy to follow instructions and troubleshooting guide.
The normal dial up will not be effected as its a seperate Modem
It could also be a faulty Modem but I would recheck actavation of line and that the system settings are correct