I have hit successfully with both elbow flexion and extension at contact, on my forehand. It does work and, yes, you tend to stick to one hitting position over an other -- it becomes an habit and, soon enough, you just forget that you'Re doing it.
I have even hit forehands with the entire array of grips that you see on the tour today: all the way from eastern to the most extreme full western you could find. I have struck with an extended elbow with virtually all those grips, except the full western (I couldn't get the contact right with it) and I have hit a bent arm forehand with all of them, except the eastern (I couldn't manage to hit a proper windshield wiper forehand with an eastern grip and a bent arm). I went from a catastrophe to a wonderful shot using everyone of these combinations, always switching for different reasons and always spending hours hitting hundreds of forehands to get the hang of it. And, believe it or not, I also went back and forth between backhands.
Every time it feels weird and, then, it starts becoming more of a second nature and there's a reason for this: knowledge and habits are social and psychological constructions. He wishes to learn to hit with a straight arm... Fine. But, get a model and practice hard.
I would also give an advice, grip-wise. Grips do not change a lot your spin production., but it does make you lean toward certain trajectories more than others. I have found it extremely annoying to swap my western grip for a continental during game play and it became obvious when I tried bringing my girlfriend into playing tennis... Try hitting softly with a western grip -- you are more hero if you can -- or try using the continental, or the eastern, for some touchy shots when you are used to a full western. It goes way out of whack because you're used to a face which is far more closed and, therefore, to a forearm which allows the face to open a lot more.
The point is that going for less extreme, such as with a mild semi-western or a very definitive eastern can ease your game a lot if you enjoy using the whole court. It also makes it easier to deal with the straight arm thing... trying to make a good contact, using WW finish and dealing with an extreme grip all at once can become really, really annoying.