Daemons are a hard army to use, but once you get used to them decent enough. A mate of mine uses a Khorne Daemons force, and he's pretty good with them, now that he's gotten used to them.
However I wouldn't be thinking in terms of beginner armies, because your friend will simply end up collecting an army he doesn't want or like.
Anyway here's a quick run-through of the forces, based on what I know:
Space Marines - Balanced armies overall with good choices of melee and ranged units.
Chaos Marines - A bit more assault orientated than Space Marines, but again a nice mix of melee and ranged.
Orks - Poor shooting makes for a point and run combat force with lots of melee units. Have the strongest general HQ in the game in form of Warboss with Powerklaw.
Imperial Guard - Shooty horde army, platoons and lots of tanks avaliable to players.
Tyranids - Horde army with balance between lots of small critters and big heavy hitting units.
Eldar - Need units to be used in concert, otherwise your dead, but nice models and colourful variety make for spectacular looking armies.
Dark Eldar - Fast but fragile, need to play lots of games to get feel of army, as well as pick your moment to strike. Old models and codex tend to put a lot of people off, but scope for conversion can deal with this.
Tau - Point and fire style of play means you want to avoid combat at all costs, nice looking infantry and tanks for the player who wants something a bit more futuristic and sci-fi.
Necrons - Arnie's army. Robot army with little variety, but some of the most effective units in the game. Necron Lord is a good HQ, whilst Monoliths can potentially stop play and hand you victory before you've even started playing (Okay it happened once that I know of, but these pyramids are still lethal in the game.)
Daemonhunters/Witch hunters - focus on combat, but interesting choices of support unit. Note extensive use of metal amongst models in these ranges.
Daemons - Potentially very deadly, but unique deployment and rules can take some getting use to, though not so much if it's your first army.