During sexual arousal, muscular tension (myotonia) develops throughout the body. It's not uncommon for men and women to curl their toes, make their hands into fists and clench their teeth. Is she enjoying sex and orgasm? That's what is most important. However, if she feels like she's limiting her pleasure or not getting the most out of her orgasms, I would recommend some tantric breathing exercises. Also, you should encourage vocalization of her pleasure. If she's uncomfortable with moaning and vocalizing her pleasure, join her in the activity, and also put on some music.

@rick - it's difficult for some people to "unlearn" patterns of muscle tension that they learned in reaching orgasm, but certainly not impossible. She would have to consciously work on changing her patterns, which would mean a little trial and error with focus on pleasure and not so much on orgasm. Good ways to start are for her to allow herself to vocalize her pleasure a little more. This helps her have her mouth open if she is gently moaning (or yelling - hey I don't discriminate!). The other thing to try is experimenting a little with her breathing. If she is clenching her teeth, there may be a good chance that she is also holding her breath. Panting or exhaling completely as she feels she is close to orgasm might be helpful.