Why would you give someone such subjective advice? That's like saying "Please use a red car!" Personally, I can't stand comfort hards. It feels like drifting on ice, but to each their own. He should probably use comfort tires for learning on, but comfort hards might be excessively forgiving for teaching good driving habits. If he uses comfort hards, he should not use them exclusively or he won't develop very good skill.
As far as actual advice, it sounds like you are just hitting the usual problems for a beginner drifter. It just takes practice. Like Brandon said, the best way to learn is by experimenting. You don't generally go full throttle through the whole corner. Try entering a drift and as soon as you get it sideways let go of the throttle. Watch how long it slides before it snaps out of the drift. How long it slides will depend on how hard you "threw" the car into the drift, or how harsh your weight transition was.
Now try again, but this time, get back on the throttle again right before it's about to snap out of the drift. If you time it right, you can catch the car. Drifting is a constant balance, you have to anticipate the car and counteract its forces. The stiffer the suspension, the faster you have to react to the subtle weight transitions of the car's body. I think 350's are pretty rigid naturally, so keep that in mind. Probably not the best learning car.