Hopefully, if your friend is doing this job for the first time, it's not YOUR house he's doing. If he wants to do this type of job himself and save some $$$, ask friends and friends of friends for someone who knows what they're doing and let him be their assistant for a job on weekends. In short, if this job isn't STARTED right, the finale will look like crap. You (nor he) would want that now, would ya???
The hard/difficult parts are starting the job, working around windows, and working around lights and spiggots and such. I suggest he look at anything that's at least .042" thick. Anything thinner will buckel. It's not worth buying. I also suggest he stay with neutral colors like gray, white, tan, beige, etc., etc., because not everyone likes green or blue when it comes time to sell the home. One or 2 people know that vinly can be painted but now you've gone from a maintenance-free home to one that needs maintenance! Also, read the warranty FIRST before you buy. If you don't follow the instructions in the warranty, you can void it.
Also, if $$$ permits, this is a good time to add outdoor electrical boxes and more spiggots. (AT least you can put in the 'accessory' now and the hardware later.) This type of info is talked about in instruction manuals. While the ladders and scaffolding and pics are up, it's a good time to clean, prime, and paint any windows, roof lines and the like. Better yet, just clean and prime the wood and have everything capped w/aluminum for a TRUELY maintenance-free home. The way I look at it, if you're gonna save some $$$, put some if that savings into quality material and some nice extras. Now, to answer your question ...
Ashland-Davis. I like this site and I like their products and it worked for me. (I don't work for them directly or remotely indirectly.) When you go there, there's a Homeowner's BOX. In there should be EVERYTHING your friend needs. And if I'm not mistaken, Home Depot still sells Ashland-Davis. The model and style I bought they didn't have in stock, so I ordered it. My order was over $2000 and I didn't have to pay delivery (which was only $40 anyway.) Get extra siding, channeling and such because you never know when a style/model will no longer be produces. Besides, vinyl can get damaged and you'll need to replace it. I got an extra 10 sq ft, plus channeling. In the future, we may want to add 2 windows, and when we do this, we'll need channeling, possibly extra siding for 'mistakes' that are made. Hopefully, you friend knows how to properly size-up a job. If not, have a contractor or 3 come to you home and price the job. Ask how much siding they'll need. Go w/that and add 10%. Heck, at worst, you may be able to save some $$ by buying the material for a pro yourself and just paying labor. In this case, they SHOULD give you a FULL material's list!!! Have the contractors price out BOTH methods, the latter is what you ask them last.