Go to Dell and look for mouse drivers for Mac OS X under Support. It should have all basic functions out of the box.
Some drivers for mice, although said to be Snow Leopard compatible, are not or not very compatible.
I have an $80 Logitech laser mouse that if I install their driver, freezes my machine after a while of use and renders it unusable until I either archive and install the operating system or erase and install same.
Such a shame that correspondence with Logitech on this issue left me feeling that they don't care and are not going to try to fix their driver.
So, make sure, if he finds and installs a mouse driver, that he backs up all before installation of the driver.
Good luck.