@jambi Also lived and vaped in Mexico he can surely fill in anything I missed
You covered it really well in your post above. I'd only add that there's no reason to try to import eliquid when there's excellent Mexican made eliquid already available via internet there, and at prices that rival even crappy budget US vendors. I'm happy to post links upon request. Gear is also available domestically, though prices for gear are generally higher, thanks to grey area laws and insane shipping and importation costs. Also, don't expect to find the latest greatest stuff to hit the market at any price, it takes awhile for stuff to get into the hands of MX vendors, but it does get there eventually. When you see something you want that's in stock, my advice is to grab it while it's available, or risk watching it go out of stock for months.
RE: shipping...even if your internet vendor offers a free "Correos de Mexico" shipping option, pay the 100 pesos and opt for Estefetta or Redpack or whatever private carrier they're offering. Correos de Mexico is, in my experience reliable, but they're also slow. I can't speak for "very rural areas", but that was applicable in Oaxaca City. Maybe Correos is a better bet in remote areas. I can see a scenario where Correos has daily service to a remote area that Estefetta might send a truck to only once a week. Your friend will need to figure that out.
The only problem I can imagine is if your friend is in an area so remote that it doesn't have an Oxxo. The pay in Oxxo option is the fastest, easiest method by far for buying anything via internet in Mexico, and it just flat out works. I've done it dozens of times and never a single glitch. If you can scan and email a copy of your receipt after you make the payment, it'll save a day in processing time for your vendor, though it's not obligatory. The bank transfer thing, where you take your cash to a branch of the vendor's bank and deposit it into their account, is also an option, but it's more of a hassle. In other words, Oxxo es mucho mas rapido than any Mexican bank I've dealt with.
Also pass along to your friend, though all of the MX vendors I bought from had Paypal accounts, they often didn't work. Paypal knows these guys are selling items on their prohibited list, I suspect. The Oxxo thing is by far the fastest.
I have a feeling that if/when the FDA .... hits the fan in the US, Mexico will modify its current position regarding ecigs and become the largest manufacturer, grey-black marketer, and importer-exporter of ecigs to the US in the world. Ecig stores will line the border like pharmacies do now, and all of us will be making runs to Nuevo Laredo for the latest gear.
Good luck!