单身老猫,please help!

请教单身老猫 and 有经验的朋友:

My father (70 years old) had a tooth filling about three weeks ago then had a root canal done for the same tooth two weeks later. When the doctor did the root canal, he cut (or removed) half of the tooth. He said because the tooth had a lot cavity. So I have two questions here:

1. If the doctor needs to remove cavity from the tooth to do the root canal, why did he fill the tooth first? Is this a fraud?

2. Since he removed half of the tooth for the root canal, now the Fake teeth can't be put back (this tooth used to be a hook for my father's fake teeth). It is too loose. My father can't eat normally. He hardly eats well. The doctor knew my father has this fake teeth and he never told us what could happen to this root canal. Actually he never told us much about the root canal. He only said one of the teeth needs root canal. He never told us which tooth need it and what kind impact will be. When we asked him what he can do to do some fix, he said we have two choices: 1) put on a crown which will cost $750 and he is not sure how long the tooth will last even we want to put on a crown. 2) He can put one hook to the Fake teeth to hook on the front teeth and he will charge $100.

We feel angry about the doctor's doing. First of all, we think this is his responsibility to make the whole thing terrible. If he told us the impact of the root canal, we have choices to select not doing it. Secondly, he even doesn't want to do any efforts to fix it.

So what can we do to get the thing be treated fairly? Where can file a complain about this? Any suggestions?

Appreciate your help!