For a well rounded group in all four roles, and 3 different power sources (Arcane, Martial, 2 Primal):
Striker: Rogue, DEX/CHA build. Skills: Acrobatics, Bluff, Intimidate, Stealth, Streetwise, Thievery.
Leader: Bard, CHA/INT build. Skills: Arcana, Diplomacy, History, Religion, Heal.
Controller: Druid, WIS/CON build. Skills: Endurance, Heal, Insight, Nature.
Defender: Warden, STR/WIS build. Skills: Athletics, Dungeoneering, Nature, Perception.
Result: All 17 skills covered, with some overlap on Heal (with the Druid being the true person for the Heal check, the Bard picking it up for emergencies despite not having WIS primary/secondary), and Nature (shared between Druid/Warden.) Hits all four roles. And likely has a bit of fun in the process. ;)
Since you said he is an old friend, maybe he just doesn't know exactly what a defender is supposed to do (Put the monsters on a hard choice between attacking someone who has better defenses, or take a harsh punishment), and instead tried to make a "tank" as defined by the previous editions (Have a huge AC, have a ton of HP) and expects the monsters to attack him because he is the toughest guy in the group.
If that is the case, I recommend you to ask him to read an old article in the WotC forums, The Art of Defending. It is NOT up to date with the final state of 4e/Essentials, but gives great insight on what a Defender is supposed to be doing.
Player expectation is also a major factor. Check out if he actually likes to be the toughest mobile rock in the field while you make all the monsters ignore his character as if he is not there. Maybe he gets angry because he has all the HP and surges, but no one hits him. Maybe he will glee with joy because he can set up easy flanks and "survive" as he desired. In any case, as long as he notices that you are not hitting him, he might end up doing one of two things: Raising his punishment because he hates to be ignored; or raising his damage because he knows he can focus on offense.
But never, EVER, bluntly say that he is playing wrong. If he is being detrimental to the party tactics, the other players should be the ones asking him to be worth his salt. If he is not playing as you expect a defender to play, but the party doesn't have major issues with that, keep going. One of the best defenders I ever played was a hybrid paladin|sorcerer that dipped in control and striking, and the DM was worried that I would not be able to work as a defender.
Best Answer
The answer to this is going to greatly depend on your playstyle. I'm currently playing in a few groups. One of them has no cleric (but has a paladin) the other has no cleric or paladin. Another group has no arcane spell casters of any kind. And another has a cleric, a wizard, a fighter and a rogue.
The group with the cleric, wizard, fighter and rogue is certainly having a much easier time than the other groups. However, the other groups are not doing badly. The group with no cleric or paladin is certainly spending more time with medical kits, and making sure there is always an escape route.
Nothing in the rules per se list what sort of roles a party needs, but for the easiest time, there should probably be:
That does not mean that you need a 5 person party, as often the person who can do lots of damage can also do one of those other roles, but from my experience those "roles" will make you prepared for almost any adventure.