Pain under the tongue could actually be any number of things. It could be something as simple as a mouth sore caused by being poked or scrapped by a piece of food. It could be caused by a mouth infection or it could something very serious such as a cyst or tumor (or even mouth cancer). It could be an abscessed tooth that could be affecting (or infecting) the gums and tongue. Try swishing some warm salt water 3-5 times per day. If it does not go away or feel better within a day or two, she should return to the dentist for more evaluation and treatment. (Have the dentist take x-rays or do a blood test--something more than just a glance).
You should look up a homeopath or herbalist in your area. A lot of things have causes which the regular MD or Dentist has no idea about or refuses to learn about. Ask around your community or visit a local health food store to acquire some contacts. You might be more than pleasantly surprised. More and more people are turning to "alternative medicine" with remarkable results of better health.
My tongue used to hurt the next day after making out with my boyfriend.