Know anything about broken golf clubs?

I am a high school golf coach and one of my golfers has broken the head off of three 3-woods and a 7-wood in the last month. She is not making any kind of violent contact with the ground and, while she does come over the top a bit and swings pretty hard, she appears to be making a normal swing on the ball. She has "knock off" clubs with graffite shafts and each time she breaks one, she replaces it with a stiffer shaft. I have never seen anything like it. Is there something about her swing that I should be looking at or are should I just tell her to quit buying lousy clubs?