As far as current day situations? Fracs have the stability (relatively), payscales, and employees are treated well by the company. The frac industry is growing by leaps and bounds whereas the airlines are consolidating, reducing pay, employees are beaten and morale is low. I think the potential for the airlines is better depending on where you fit in seniority-wise. Someday at an airline of choice you could have 18-20 days off per month flying day trips, or whatever type of schedule you want, but in fractionals youll be chained to the 7/7 sched forever (with few exceptions like the NJA 18 day/15 day scheds). If someone is hired at a major airline and is under the age of about 30-35 then they are in running for a good airline career, if they are willing to take a gamble/furlough at some point.
I think this is a tough question to answer and I am trying to figure it all out for myself as well. Good luck to your friend whatever they decide.