The most famous barrio is El Poblado, close to parque Lleras, nightlife, restaurants, but with VERY steep streets. Nearby is Las Palmas with Intercontinental. Further out from Poblado is Envigado, which is very peaceful and nice, but a bit far. A good option is Laureles, which is not as fancy as the other ones, but still very nice, with nice restaurants, laurel-lined streets, and very walkable as it is relatively flat.
But regardless, your friend has to physically go to MDE and see the properties and the city before buying anything in particular.