I buy all my flavorings from "TFA" or
The Flavor Apprentice and have been pleased with their flavors thus far. I'll tell you DIY juice is kind of like making beer though. It'll take LOADS of experimentation. The hardest thing to account for is that depending on the flavors used it takes 1-2 weeks of sitting (also referred to as steeping time here) before you get the actual flavor. I've made up flavors that kick butt 2 days out of the box and then 5 days later go all horribly wrong.
Also, please make sure your Dad is aware that high concentrate nicotine is poisonous and dangerous. Use gloves, etc. A little known fact is back in the day concentrated nicotine liquid was used as a pesticide. Spill some 100mg/ml nic on your skin and you've got a problem. I'd suggest he start with a lower concentrate in the 24mg range and dilute from there. It is less cost effective but much safer.
The last bit of advice would be, forget about the nicotine all together, and just get a bunch of 10ml bottles (glass are best, next hard plastic, then the soft) and mix just VG/PG and flavor. That way he can makes up a bunch of small batches till he gets the flavor he likes and then figure out the right ratio to add NIC in. I'm a "cheater" insofar as I'm no good creating flavors from scratch. So I always try out new ones and then try to replicate/tweak them for my personal use.
Oh, I forgot, My Freedom Smokes (no affiliation with our company) has a lot of great DIY gear!
