Best thing to do is go to Crucial.com and use thier memory configurator.
They gaurantee compatibility.
Crucial dixit: http://www.crucial.com/store/listparts.aspx?model=XPS%20420
Guaranteed-compatible memory: 1,8V and 2,0V memory modules ?????????
If you install 1,8v modules is not possible to install 2,0V. If you have installed 2,0v modules and you install 1,8V, the first ones won't work if you set 1,8V in bios, and you can damage the second ones if you set 2,0V. So, firstly, He must see the voltage of his modules before buy anything.