I think a big factor in this isnt just what nicotine strength you choose, but how much juice the person vapes. If you are like me and you only vape about 1.5-2ml per day, then you can probably get away with safely raising the nicotine level, although I have actually lowered mine from what I tried to start out with, and I'm down to vaping liquid that between 9mg - 15mg/ml.
I think thats very important. Its just like if you have a back ache and you have 1000mg Motrin in your medicine cabinet, vs 250mg Motrin right beside it, obviously you should take less of the higher dosage pills than you would take of the lower dosage pills. It should be the same with vaping because nicotine is something to be taken seriously.
I really dont want to get addicted to a higher dosage of nicotine than what I was getting from my 1 pack per day of cigarettes before. If anything, I want to start lowering my nicotine dosage to where I'm using less nicotine than when I smoked.
If it seems he will be vaping a few ml's a day, then he should be vaping a lower nicotine liquid. I smoked for over 20 years, and I thought I would need to start out REALLY high with my nicotine dosage, so I actually ordered my largest bottle of e-liquid with my 1st e-cig kit in 36mg/ml! I immediately discovered that was WAY too high! I very quickly began ordering and mixing my liquids down to 9-15mg, most of them 12mg.