Law regarding services NOT rendered.?

My fiancee went to a dentist for some work. She went in for a consultation and they quoted her a price and an appointment date for the work. When she declined to have the work done, they charged her over $1800. Since they didn't do the work she, naturally, didn't pay them. They then sent it to collections and won't stop calling her about it. We are in Dallas, TX. Is this legal for the dentist office to do that? Help!