Firstly, congrats! The moving of a family member from smoking to vaping is a huge thing, I know because I have been able to do it 3 times. You have tons of great advice so far but I will add my .02
From my experience 2 things affect the likelyhood that some one will fail or succeed with vaping, juice and equipment.
My wife is the prime example, I started her on a kanger evod starter kit with a tobacco flavored juice called "tobacco blend". She hated the flavor and didn't get very good results from the evod so she barely used it, she would still smoke and try to vape but just couldn't do it. After a week or two of this I finally convinced her to go to the local store with me and sample the juice, she found one she liked and was then more willing to vape, then I spend $8 for a dual coil aspire tank and then she was set, this was what she needed to get off the cigarettes, a good tank and juice she likes. We ended up going with 18mg juice per the recommendation of the store owner( I also use 18) Once she had the good set up she decided that she would quit smoking completely once she ran out of cigarettes, being that she was now vaping more than smoking it took her about 2-3 weeks to get through the 3 or 4 packs she had and then she stopped. Since then I have taken her back to the store and she picked out her own set up for the first time and she got an ego twist vv battery and a kanger pro tank 3, she loves it. I was also able to acquire her an RDA with a mech mod which she uses when the cravings are really bad. I will also add that her father whom I was also able to convince to try vaping has a similar story but he spent the money up front and got a nautilus aspire tank with a vv battery and he was happy but he had been smoking so long that he has to use 24mg juice to satisfy but he has been an ex-smoker for about a month now.
The moral here is you get what you pay for and on the set up you should not skimp. Make sure you have at least a variable voltage device and as far as atomizers I would assume that neither he nor you are looking to build coils so I would say go with something in the high end, like an aspire nautilus, this device allows you to adjust your airflow which can make it hit harder when you want or if it's too much you can back it off, this paired with a variable voltage battery and you have the ability to really tailor it to what you want at the time.
The final thing is juice, judging by the fact that you said they have "medium" and "high" tells me you are probably buying cheap juice (though I may be wrong) most of your better juice brands rate the strength in numbers, not levels.. In other words you can get 6mg,12mg,24mg, etc.. The number represents the mg per ml of the juice, most quitting smokers will need 18mg, but some may need 24mg, and others may be ok with 12 or 6, it really depends on the person. The best advice here is to buy a good atomizer and fill it with 18mg, then use the one he has now as a spare and fill it with 24mg, when the cravings are bad tell him to switch to the 24 then once it calms down go back to the 18. I personally use
ECBlend Flavors | E-Liquid | Personal Vaporizers, they have an awesome selection and a great price. The right flavor of juice can make ALL the difference so I HIGHLY recommend finding a local vape store where you can go and sample as many flavors as you want, as I said above this is what really turned the corner for my wife. In addition at a local store you can touch, feel, hold all of the cool thingy's, having a new set up especially one that he picks out himself can also be a major motivator in switching over, sounds strange I know but it's true..
Either way every cigarette that he does not smoke is a win, a big one. The cravings will probably not ever go away but they will certainly become so manageable that he doesn't even worry about it. One other thing that I and my wife found is that the longer you vape the worse cigs get, after a few weeks of vaping a cig is so nasty and makes you feel so bad that you just don't want one, when this happens you are there after a few nasty cigs you'll finally get the idea and just put them down, I have seen it 4 times in person now! Coughing when quitting is completely normal, smoking leaves all kinds of goop in your lungs and by continuing to smoke you just keep putting more in there, once you stop the lungs have a chance to clean themselves, and they will! He will cough like crazy and eventually will cough up a black nasty bunch of junk, this is inspiring because that stuff is no longer in there and at this point you realize just how much good you are really doing for yourself!
One final thing, when it comes to ratio of juice, typically I have found that most people who use 100 vg either drip it or have a terrible reaction to pg, but I think the average person uses either 50/50 or 60pg/40vg, I know that varies widely among people but that's the range most juices are sold at. With the website that I posted above you can specify your ratio, I would recommend once you find a flavor that works order a few bottles in a few different mix ratios and nic levels. There is a lot of experimenting in the beginning but once you get it right, it's great. I say all of this because I have not had the best experience with 100% vg in a tank style system, it is extremely thick and will clog up coils in a hurry and it also does not burn as quickly as 50/50 so you don't get anywhere near the performance you need from it. I think using a better juice will make a big difference on his ability to handle PG. The last word of advice, don't buy juice from the local truck stop/wal-mart. Always buy it from a vape store or an online retailer, this will insure you're getting the best ingredients and quality, that alone is worth the few extra dollars!
Best of luck, and remember this is chess, not checkers!