One2Free prepaid and an iphone 6 should be all that's needed to start a life in the most free economy in the world. Their network is robust and the data speeds are fast (4G).

Just a few extra tips for OP, relocating family member might want to consider porting her/his current U.S. mobile number to Google for a small fee (I think it's $20) and then friends/U.S. contacts can continue to dial his "current" number for the 2 year job assignment in HKG. The tech. forum has useful info & of course, Google for help.
Many public places in HK, including the MTR (subway a/k/a Chicago's "L" below ground & elevated outside the city's core, especially the newer lines) will have free WiFi access, so a newer smartphone is almost a must - 75 years old grandpa & grandma got them to easily "WeChat" each others. As said already, there are no "contracts" for devices, you basically paid full price upfront for the smartphone, pop in the Sim to activate & then "go" - iPhone is a rather "expensive" investment for some but there are plenty of choices & alternatives - keep in mind that depending on the carrier selected, a U.S. specs. smartphone might lack some of the bands to take full advantage of 4G LTE functions (i.e. 2600 Mhz) - might want to consider the Android OS as the choices will be mind-dazzling.
I opted for a China Mobile HK prepaid Sim with a local # 5 years ago & continued to use that as a backup line when we travel, as it's mostly for voice calling & map/simple Google lookup - data is slow - on the smartphone - unlike some other carrier plans, I can refill that easily from the US every 6 months, keeping the same # with a $7 USD refill every 180 days using PINS or online via CC from the US.
Otherwise, stick with One2Free - start with a prepaid plan for 30 days to try out the service and once happy, move to the regular plan if desired. With Skype calling and old # ported before leaving the US to Google Voice, and inexpensive IDD calling from HK to the US to keep in touch with "home" will be inexpensive and frugal.
I'm sure I missed/left out some of the recent changes since our last trip there - so let the local experts based in HKG spell out the rest in details.

Prepaid cards are readily avaliable at convenient stores, smaller phone shops and operators shop. If you don't get the prepaid card you want, just try next shop. I think unlocked version of iPhone in US has similar price as in HK so it won't be too bad. China Mobile doesn't work for earlier version as they only started offering iPhone recently (from iPhone 5s as I recall), and their mobile network band is newer one.

To be honest I won't rely on the free or paid wifi access at public places, I got free CSL Wifi bundled with my plan and it's way slower than my mobile connection, because of the students doing their homework at starbucks or other coffee shop

In Hong Kong for more reasonable priced phone, I will recommend look at Flagship - 1 generation (i.e. LG G3 / Sony Z2 / Samsung S5) as the price is reasonable and you are not missing lots of functions.

I've had great success with prepaid sim from China Mobile. Available from most 7-11's in HK.

www.hk.chinamobile.com/en/prepaid_services

Purchase was made easy when I brought in a printed page from their website showing exactly which sim I wanted. The first 7-11 was out of stock. I walked 1 block to the next 7-11 and got exactly the sim I wanted and it worked perfectly and without registration!

It was easy, cheap, and functional.

I agree that it works, as a matter of fact I use it for my China number using dual number service. The problem gets close when you are really working here, rely on your mobile Internet and that China Mobile is currently a Virtual Operator using CSL service when used in 3G, which isn't unusable but you hates in when you need the speed (all the data traffic are prioritised).

I really can't thank you all enough. I think I finally "get it". I especially appreciate the very specific information and comments on phones and providers. And, hearing from multiple posters about their personal experiences gives a fuller picture of what things may be like when there. Having a printed web page for the card he wants sounds like a great idea. We are on our way to success, I believe! Thanks again.

I can't believe this is true

Let's stick with one carrier - my carrier - 3HK (I don't get much choice in the matter given where I work and live. They have some sore points in customer service but When It Works It Works).

And let's say 500MB (in a recent local Youtube clip 500MB is associated with working poor in HK - let's say I'll be dead in a week with 500MB)

And let's assume a 64GB iPhone6, not Plus

Min plan price: 3,980 + (188+12 tunnel and admin+36 compulsory VAS)x24 = $9,644 over 24 months http://iphone.three.com.hk/website/e...hlyplan_n.html

Buy phone and go onto SIM-only plan: 6,388x0.95 (I assume you can get 5% off Apple official price through use of certain card rebates) + (138+18 tunnel and admin)x24 = $9,812.6 http://www.three.com.hk/website/appm...&pageid=215201

Buy phone and go onto prepaid-SIM plan: 6,388x0.95 + $50x2x24 = $8,468.6 but your relative has to pay for voice separately http://www.three.com.hk/website/appm...51002&lang=eng

>> I think unlocked version of iPhone in US has similar price as in HK so it won't be too bad. China Mobile doesn't work for earlier version as they only started offering iPhone recently

Used China mobile on iPhone 4S and 6 with data just last month. Yes it's a crappy network, the 3G is outsourced, and every connection attempt is delayed - I suspect if you buy the top speed LTE package it would be better. But you do get what you pay for.

CSL/One2Free/1010 is better, regarded as the best network for data (as per phonehk.com), and I use them on most of my numerous visits.