Aha, you're from my neck of the woods! I therefore can pontificate from many years' experience.


Please leave all cotton at home (it has its place in hot dry areas like the Grand Canyon in summer, but not in the Northwest), and urge your friend to do likewise. Cotton absorbs lots of moisture and is much slower-drying, not something you want in the Northwest unless you really want to die of hypothermia (think evaporative cooling). Jeans get sopping wet and HEAVY just from hiking our brush-lined and mostly overgrown NW trails which drip with heavy dew every morning even if it hasn't rained for weeks. They may stay wet all day. (Think about it--what is the last item out of the clothes dryer?) Cotton or part-cotton sweats are no better. Synthetic sweats (basically fleece) will be too warm. On the other hand, those nylon/polyester pants (zip-offs or otherwise) will dry from your body heat in 30 minutes, even after wading through a thigh-deep creek.

The nylon or nylon/polyester pants do keep out some of the wind, and for me are plenty warm enough down to the low 30's (F) without a base layer underneath, as long as I'm actively moving. However, as soon as I stop I get my base layer bottoms on in a hurry when it's cold!

ChrisFol has pretty much covered the basics of layering. I would like to add that rain pants (in addition to a rain jacket) are essential in the Northwest. While on a warm day it doesn't matter if your nylon pants get wet, if it's a cool, windy or foggy day you need the protection. It doesn't have to be raining for that waist-high brush to be dripping wet! You can find lots of good stuff at the big box stores in the athletic section--nylon track pants, breathable synthetic shirts, decent socks (even merino wool). I would never buy fleece at an expensive place like REI--there's lots of good fleece to be found a lot cheaper at the big box or thrift stores--just inspect it by looking through it at the light to make sure there are no thin spots.

Just as a sample, this is what I wear or carry:

WORN:
Trail running shoes
Socks, merino wool
Gaiters, low (to keep gravel etc. out of shoes and mosquitoes from biting through my socks)
Hat for sun (I use one of those dorky Sunday Afternoons hats because I burn easily and am allergic to most sunscreen)
Long pants, nylon (may or may not use zip-offs; I rarely wear shorts so take zip-offs only if the trip has lots of stream fords) or * for really cold weather, military surplus tropical weight wool/polyester pants (bought in mid 1980's and still not worn out!)
Shirt, GoLite C-Thru or Capilene 2 zip-T (any lightweight, wicking synthetic will do)
Sports bra (obviously not needed for males!)
Briefs, Ex-Officio

CARRIED: (those starred * are for cold weather, added to the others except for the hiking pants and base layer bottoms)
Socks (Smartwool PhD light cushion)(1 pr)
Base layer bottom (Capilene 2 in summer or * 4 in cold weather)
Sleeping socks, fleece (luxury item; my feet love 'em!)
Jacket, Montbell UL Thermawrap (or 200 weight fleece)
*Vest, 100 wt. fleece
Balaclava (polypro fleece)
*Headband, merino wool
Gloves, liner, polypro knit (I'll switch to light wool if I can find it)
*(Cold weather note: gloves need to be thin enough that you can do camp chores such as cooking without taking them off)
*Thicker fleece gloves or mittens
Rain mitts, waterproof (and breathable if you can afford) (Plastic bags will work in a pinch)
Wind shirt, lightweight (a thin, unlined nylon jacket with added DWR treatment works, but if you're looking at big box or thrift stores, they'll have a flannel lining that you'll want to cut out)--basically a very light, wind-resistant, somewhat water repellent yet highly breathable layer. Since I got one, I'm amazed at how often I use it! I even use it for bug protection, particularly when biting flies are around.
Rain jacket
Rain pants

One important principle of layering is to keep your body warm but not so warm that you sweat. The scientific term for this is "thermoregulation." That's why multiple thin layers are better than one thick layer. It's especially important to avoid sweating in cold weather, because you don't want your insulating clothing to get damp and lose its insulating power. On the other hand, you need to add layers as soon as you stop to avoid getting chilled. I call this removing and adding thin layers as needed the "onion" principle.

Example: Last fall I was hiking a steep (up) trail out of the Entiat River valley with the temps in the 30's with some wind. While hiking, I wore my base layer top, the wind shirt and the wool headband, and was very comfortable--not cold but not sweating--as long as I was moving. The instant I stopped to rest, out came the insulating jacket and gloves and, as it got colder, even the rain jacket. When I got to the camp site (it was already below freezing and the wind was blowing harder out in the open), out came all the layers, even the rain jacket and rain pants although it wasn't raining, all of which I changed into the minute my tent was up. If I had continued to hike in the colder conditions in the meadow, I would have added the lightweight fleece vest under the wind shirt.