Well, let's assume that all three practice pretty good gardening techniques so then, let's bring in another controllable entity into the picture and that is soil pH.
But first, let me say, NW15, that I have been to your area several times (Aneheim and areas south and east of you) and I spent a lot of off time strolling city and county parks as well as driving up and down many residential streets looking at peoples lawns and flowers. And the soil seems to be quite sandy with pretty good drainage--which is good. And temperature over 90'F are a rarity. Further, there's often a slight marine haze in the air which filters the sun so the ground temperature's don't get terribly hot. These are all good things that work in your favor. I won't mention how much time I spent in all those good coffee and doughnut shops you have in your area.
Now, let's talk about pH. We all know that most all lilies do their best in a soil that's slightly acid--a pH somewhere less than 7.0. This produces nice healthly bulbs. Asiatics and Orientals, especially, need this lower pH. Trumpet types can tolerate a slightly higher soil pH but they, too, would benefit with a pH slightly less than 7. California has a reputation for having a high soil pH, as do sandy soil types. So growing big healthy bulbs is a bulbs best defense against many things. When you say your Asiatics languish in a downward spiral over a period of three or four years seems to indicate your soil pH is high, while at the same time your OT's do just fine--they would, as expected. So if you could, get your garden soil pH checked and if it's running a little high, as I suspect, bring it down a little gradually by adding Sulfur Crystals or Calcium Sulfate (but not Aluminum Sulfate)
Now, I suppose it could be said that I contradict myself in the case of Fusarium fungus and Basal Rot when I suggest lowering the pH. And that's because in doing so, one is creating a somewhat more favorable environment for the fungi because they like a more acid environment. But the facts are, they will affect a damaged or unhealthy bulb regardless of pH when they are present in the soil. And going full circle with this, you can see that both Orientals and fungi like the same soil pH conditions. And that's where good proactive measures of fungicides comes in handy.
If you correct your soil conditions and use fingicides successfully, your neighbor and friend should do the same. If they don't, then refrain from trading plants until they do. And always be aware of the fact that you do have these wet, winter seasons when good drainage is crucial as Leftwood mentioned.
