Rancho PV. The nice part of PV peninsula are PV Estates and Rolling Hills. Both have a strong Architectural Art Jury and an impeccable architectural guideline in placed to prevent poor taste homeowners from butchering their own homes. Most custom homes today appeal to Chinese and Persians because they like those cheap foam trimmed palaces. Unfortunately Rancho PV was created much later without the sophisticated guideline in place. The last 3 decades greedy developers built some of the most awful projects along the ocean facing cliffs including those Persian palaces and Trump's resort. Many parts of Rancho PV suffered from land slide due to subsurface drainage that weaken the earth and caused substantial land settlement. East facing part of RPV is near Maraleste and anchored by San Pedro. The view of industrial wasteland is not much of a view during the day but the nighttime view is spectacular when the city light and industrial yellow fog lights lit up the sky.The part of RPV that you are referring to is not that far from the freeway unlike the better parts of the peninsula. However, the closest freeway access must pass through the filth of the industrial city. For this reason RPV is out. The local Asian hub is in Torrance and Gardena. Both are just OK and extremely outdated.
Malibu is only accessible by one road and that one road has been under construction since the 60's. The view is spectacular but not very private. Not much of a nighttime view with the pitch black ocean but the Santa Monica pier lighting is nice and an iconic view. It is busy and full of tourists. It is the costliest to build there because of the local comp. Coastal Commission will limit both the size and freedom of architecture. I just don't see the traffic congested location and the lack of opportunity for a single story accessible home fit your friend's desirable wishes.
Obviously your friend wants to build new and the only part of the foothill that still has vacant land is the very north of Altadena / Pasadena. Few built there for a reason as land is scarse people are willing to take greater risk by building toward the hills where the fuel is thick, fire danger is heighten, mud slide is present, and wild life encounter is common. The location north of Altedena is not great at all. Crimes and burglary in the hills are common. I don't think your friend want that an this is the reason why he insists on a gated custom community. It is vibrant, cultural and a short distance to the very best architectural landmarks, cultural activities and Asian activities. It is very hot toward the base of the hill and a place where the air is stagnant and smoggy. I would recommend a no for this site.
La Canada Flindridge and Montrose are extremely quiet and both are destination community where there is no cross town traffic. both are quite hidden. It is evident that you know very little about them as most of the population. The street pattern are windy due to the topography. It has many valleys and knolls. As a result water get collected at the low points giving this city very lush and dense landscape neighborhoods. There are a lot of shade in the neighborhood from mature trees and deep shadow from the hills. Not far from there is the Descanso Garden. Architecture there is as pedigree as San Marino but without the crosstown traffic. This is my top pick for tranquility, single story living, competitive construction cost, compatible neighbors, good architecture, limited traffic, excellent schools and proximity to Asian amenities.
Pacific Palisade is also another excellent second choice. The recent lots with view were in the zone subject to landslide right above PCH. For years no engineers wanted to validate the remaining lots. This is the most architecturally diversified community and many varieties were excellent. The surrounding traffic is just too much leaving the community. Once inside it is very quiet and exclusive. I also like the destination location of Palisade. Proximity to Asian amenities is too far and the Japanese town by Olympic Blvd is treacherous to get to. The Coastal Commision comment gave away the lot location in Palisade. The terrain may be challenging and costly to engineer and build. Drainage hydrology would be scrutinized by both the neighbors and city.
Bel Air sounds great if your friends like to throw elaborate partie at their house. Ostentation is not what your friend is looking for. I don't need to elaborate.
Irvine is definite No for custom and single story. This is not a place of tranquility. The city roads are congested. The freeway is gridlock. Rent here to be close to work and Asian amenities but build the custom home elsewhere where your friends can enjoy during the weekend or the days he calls in sick. Hopefully he does not call in sick to let his employees know.