As a woman, I can definitely say extended time of giving oral can hurt the jaw. (Sorry if this gets graphic...) This is especially true if he has exceptional thickness, just the way one must hold their mouth open does strain the neck and jaw muscles. Other than exercising the muscles there's not too much that can be done to change the pain she's experiencing while performing her current technique. However, she could try changing her technique. It really all depends on where you're most sensitive. If rather than going down on the whole thing, she concentrated more on licking, tongue-flicking, and kissing your most sensitive areas, interspersed with smaller amounts of full insertion, she may find it less painful while you should still find it pleasurable. You can suggest this to her, and make sure that you tell her you enjoy receiving oral, but don't want her to be uncomfortable.
Also, maybe try changing up your position. Standing, sitting in a chair, laying down, etc.. this way she can change her position and reduce muscle strain.
Don't forget to reciprocate too! If she's not gettin any, she'll be disinclined to give you any. ;)
Hope this helps!
