Well the 400D is the older Rebel series camera body so honestly any rebel series camera in the Canon line would easily replace it. Of course there are higher bodies that your friend could upgrade to if they have the budget and some second hand options as well. It depends if they want new or will consider second hand as well as to how much money they can spend.
They can't get another branded product since it won't work with their lenses* however if they only have the kit lens from the 400D then they'd be free to consider other ranges such as Nikon if they wished - really depends upon them, what they currently own and what they want to do as well as their budge.t
*without fiddling with adaptors that might not enable all features of the lens