If there is any metal installed in the body except gold and metal singing bowl therapy should not be done.

When sound vibration get contact with any metal, they also vibrates. As a result they try to disconnect from the body. Even if there is any unnatural metalic things or any harmful substances in the body, they also try to disconnect from the body and come out which purifies the body. There is no any specific harm if you do singing bowl healing if metals are installed in the body, it just tries to disconnect and remove from the body.

Since we are talking about metal on the teeth, they also get vibrated. So, it is suggested not to do head therapy, facial, neck and shoulder therapy for those people since it is very close to teeth.
Rest of the healings which are little away from the teeth- like foot therapy, thigh, leg, hips, back therapy, chakra healing, aura cleaning etc. are okay to do.

Singing bowl always try to improve your true nature, inner doctor i.e. immune system. It tries to remove unnatural things from the body. Disease is also unnatural. Stress, tension etc. are also unnatural. So, singing bowl try to remove it.

Singing bowl always tries you to take your original, natural and pure state as you were born.

If the therapist have installed any metal in the body, or on teeth, can they heal?

Yes, they can since they are trying to direct the sound vibration to the healee, not themselves.