The tongue is responsible for maintaining breathing airways, moving food around in the mouth, swallowing and talking. It plays no role in moving the jaw. With bruxing, clenching or even headaches, it is quite common to use the tongue as a crutch to try to support the jaw and neck, which is an undesirable compensatory movement.
Your patient (and your good self) should be able to isolate your tongue function from their jaw muscles by placing their tongue on the roof of their mouth and then opening and closing their jaw. If this is achievable, then progress to lateral deviation left and right. The tongue needs to stay on the roof of the mouth, if they can’t do this, they need strategies to help break down their parafunction.