I am sorry for your loss.
You of course will need to decide for yourself if smoking is better than vaping, I have been following this since 2009 when I stopped smoking and started vaping myself. I did my due diligence first by reading everything I could on both subjects (smoking/vaping) in order to make my own informed judgement. Others have already provided some information and links, there are many here and on the internet I would urge you to investigate.
I can't help but note that your friend was a smoker, and it is proven that smoking can cause cancer. It is also known to the FDA that PG does not. I wonder why you would suspect his vaping over that simply because that is what he was doing at the time? There are things we are exposed to either by ourselves or environment or just bad luck that can affect us in very bad ways a long way down the road after the exposure. Knowing some people here that vape well in excess of 30-60ml or more a day (several bottles) I would think we would have heard of vaping related issues since 2007 by now if the long term effects are killing people, I know BIG TOBACCO sure would LOVE to spread a message about that just to bolster their profit margins. Nobody will make me believe if there was even a shred of real proof that vaping was dangerous that big tobacco wouldn't be spending tens of millions of dollars shouting it from the rooftops.
In the first weeks after switching to vaping from smoking you will likely develop a cough, not from the vaping but from your lungs trying to clear out all the tar etc from the cigarettes. This can happen if you simply go cold turkey also.
As far as the burning in your throat from vaping, I would ask what type of device are you using, at what power level/temperature/voltage/wattage etc, what type of tank/coils are being used at what ohm reading, and what e juice are you using, flavor, mg strength, brand, vg/pg composition etc.
With that information we could make an educated guess as to what might be causing the burning, if the vapor itself is very hot...that is a first guess. You could also have a sensitivity to something in the e liquid itself, the nicotine level could be too high etc. Granted you would probably also be having light headiness or nausea/sweats if you are overdoing it on the nicotine but I only mention it as it is possible. A self test you could perform if you have stopped vaping and restarted smoking would be to wait until the cough goes away, and stop smoking cold turkey for a few days to a week and see if the cough reappears.
While we are at it, just because something is used in something that is not safe for us to consume, does not mean that it is bad for us by itself. Anti freeze is not FDA approved to drink. Propylene Glycol however is FDA approved and in so many products we use every day I would think we would be hard pressed to avoid it.
Some informational links to browse:
8 Foods That Share an Ingredient With Antifreeze
CFR - Code of Federal Regulations Title 21
Antifreeze - Wikipedia, the free encyclopedia
Propylene glycol - Wikipedia, the free encyclopedia
http://www.fda.gov/Food/IngredientsPackagingLabeling/GRAS/SCOGS/ucm261045.htm
Given what is available today, I honestly believe that vaping is MUCH safer than smoking. Especially if you make your own liquid or know what is in it (buy from reputable / tested sources).
I also believe it is safer to simply not do either.
Given the choice of smoking or vaping, I chose vaping back in 09. I also had a cough the first week-10 days after switching, it was not the vaping it was the cigarettes. How do I know that for a fact? Well I stopped vaping after about a year and half, because I had tapered my nicotine usage down from 16-18mg, to 12 mg, to ..0 mg in the first 6 months and was able to take it or leave it after that. In the past few months I started vaping again, because I do enjoy it at times. Guess what, no cough.
While I am on the subject of stopping cigarettes, My sense of smell and taste improved, mt breathing improved, I stopped getting colds that turned into the flu that turned into bronchitis all the time. But you will need to decide what is your best course of action for yourself.
I simply do not want anyone making snap decisions without the actual facts / best information currently available.
Good luck, and best wishes.