One exercise I have found helpful with students in a similiar situation as your student is the following exercise. I believe it is one of Arnold Jacob' exercises:
Goal: to develop ease and security in tone production.
1. The player must allow the air to vibrate the lips with utmost ease.
2. It is as though someone else besides the player is producing the tone.
3. The player will think about three areas:
a. Take a full relaxed breath
b. Mentally hear the desired pitch
c. Produce an unforced wind column across the lips
4. The player must visualize embouchure motion as in the manner of reducing the open distance between the thumb and index finger, as if picking up a small object. This is critical.
5. To produce this skilll use the following 12 step exercises on the following pitches: C4, G4, C5, E5. G5.
12 Step Exercise
Step 1 trumpet (no mouthpiece)
Step 2-6 mouthpiece only
Steps 8-12 trumpet with mouthpiece
1. Take a full breath and blow a steady, unforced column of air
through the leadpipe.
2. With lips over the mouthpiece, repeat step #1. Unforced air is a must. Ease - create no resistance.
3. Loosely form embouchure (only blow air - no pitch produced and repeat step #1.
4. Start as in #3 but midway through exhalation bring lips together to produce the desired pitch (C,G,C,E,G). Remember to think the sound of the desired tone. Don't make the lips buzz. No sound is better than unforced sound.
5. As in #4, only now attempt (just by using air release no tongued articulation) to produce the tone immediately. Remember no force.
6. As in #5, only now use tongued articulation to aid security and precision.
7. repeat #1
8. Repeat step #2 (steps 8-12 trumpet with mouthpiece)
9. Repeat step #3.
10. Repeat step #4.
11. Repeat step #5.
12. Repeat step #6.
Any step can be repeated as many times as needed.