Your friend needs to replace the supplied USB ADSL Modem with a combined "Wireless Router & ADSL Modem", such as the Netgear DG834G. (A USB Modem cannot be used with a router).
Daughter can then connect to it "wirelessly" and the Mac in the shed should be able to get a good enough signal to connect wirelessly too. If not, can he run an ethernet cable from the house to the shed?
A USB dongle for the Mac should be fine, I've just installed a D-Link "DWL-G122" USB adapter in a MacMini running OSX 10.4.8