I do have a pretty good business because I have diversified. By being in the training, charter, management, and sales markets, I don't have to rely on one thing (ie charter) to make a buck. If the charter is slow, I am usually training someone. If the training is slow, I pick up some charters or sell some aircraft.
Money CAN be made with 135 ops, but it is tough. The toughest part is buying the plane and then trying to get your 135 Certificate. You need the plane first, then you go to the FSDO. Count on at least 6 months to a year or more to get your Cert depending on the FSDO. That is a long time to be floating an aircraft loan and insurance.
Your friend's problem is going to be insurance. New planes are great, but they carry high hull values. Most customers will choose a similarly priced Seneca II or III over a New Saratoga. The direct operating costs (fuel and oil) are close, but those aircraft have lower hull values, so less insurance cost, therefore less cost passed onto the consumer.
Feel free to shoot me an email or call if you want to talk further.
Michael