This is a little more than what you asked, but it will all be helpful in helping her complete her class.
Have her take her snorkel off her mask and have her put on her weight belt. With her mask on the pool deck, her snorkel in her hand and her weight belt on, have her get into the shallow end of the pool. She should be where when she stands she is no deeper than chest deep, waist deep is even better. Have her put her snorkel in her mouth and breathe from it. Now, she should bend over and breathe through her snorkel while she dips her face into the water. If she is comfortable, have her stay awhile until she feels natural. If she has difficulty, tell her to think about drinking through a straw while she practices the excercise. Remember, she can always stand up straight to end the excercise at any time. If she needs to, she can slowly build up her time until she feels comfortable.
Next, practice dropping underwater, allowing the snorkel to flood and coming up to blast it clear, leaving her face in the water. Have her continue until she is comfortable.
At this point, she has accomplished three important things.
1. She lowered her anxiety level by working in a no risk (shallow) environment.
2. She became an expert in no mask breathing.
3. She knows she can breathe with a flooded mask.
Now, have her put her snorkel down and pick up her mask. Have her put it on and drop down to her knees. Have her break the seal just enough to allow a small amount of water in. Clear the mask. Do not pull out on the bottom, merely push in on the top while looking up. The biggest mistake most people make is not looking up. The second most common mistake people make is not exhaling through their nose. Tell her to keep her mouth closed. Repeat this until she is comfortable. Don't let her forget to stand up and catch her breath between clearings. Now, increase the amount of water she allows into the mask to about ¼ flooded. When she is comfortable with that level, move to ½, then to a fully flooded mask. When she's reached the point she can clear it easily every time, see how many times she can clear it on one breath. After she is at this point, clearing her mask and no mask breathing will be easy. Her issues with nose breathing will be over. She will have reached these milestones in a step by step manner that was easy to accomplish.
Remember, she can always stand up if she feels uncomfortable.