I was in the same situation with a friend of mine coming from a D&D 3.5 background. If your friend is expecting to play the sort of necromancer who controls a horde of undead, that's not possible in 4th Edition with the RAW (Rules As Written). The game is balanced around each player getting 1 attack on their turn (barring powers that allow more than one). If a necromancer had a host of skeletons, for instance, they'd suddenly have far more attacks than other players. This would break the economy of actions.
You can build an effective necromancer using the Wizard class. They have a daily spell at L5 where they mark someone, and when that target dies, they rise again and follow the wizard as an undead servant until they are destroyed. They are, however, a minion. So they're not extremely combat-capable. But it's easy to flavor some of the wizard powers to be more necromantic, and play up the feel of a dark practitioner. I know there's a 'Witch' build for wizards now, which I haven't looked at; that might have more material for you.
Alternately, if you're good with rules tinkering, there's no reason you couldn't tweak a class to have undead minions. You could adapt the ranger 'beast companion' stuff to use it.