I concur with what most people are saying here
Europe uses 240V @50Hz mains power while the USA uses 120V @60Hz mains power so the power converter is vitally necessary (and you'll seriously damage something if you don't convert it) Alternativly you could get a new European Power supply for the 360. the power coming out of the supply and going into the main unit is the same for all regions, it's just the mains input that's different.
As for Video in Standard Definition Europe uses 576i @50Fps and USA uses 480i @60Fps... for HD standards they use the same resolution and I believe all European HDTVs are required to support 60Hz which would make them compatible with USA consoles.
Also note that your friend will likely not be able to access portions of the USA Xbox Live Marketplace because they filter by IP region not by where you specified your home country... Simply connecting from a European IP pegs you as a European even if you told the system you're an American.