Possibly small claims, where you don't need a lawyer.  Still, if peer
review is open to you it probably will be less time consuming and
traumatic to your mother.  While it is run by the dental society, at
least in my local society the peer review committee actually finds at
least partially for the patient in the majority of cases.
    It is extremely unlikely that a lawyer working on a contingency basis
(the usual way this is done) will take a case this small.
    Unless you can clear this up yourself with the dentist (which given the
facts as you've stated them sound doubtfull) she'll be better off with
another dentist.

Steve

Thank you for your insight. Why is this not a lawsuit situation?  What
recourse would my mom have for treatment if the dentist refuses to talk
to my brother, etc?  We can report him to the associations and
insurance company, but ultimately how will my mom get her tooth taken
care of?  She has been in pain for months.

    There are two issues: the financial issue is one, but the bigger is the
negligent care your mother received.  Specifically, not only the
interruption of work but especially not making provisions (temporary
crown, for example) to at least make her comfortable and minimize
drifting of the tooth while the insurance situation is straightened out.
    In many states it is not legally permissible to discontinue treatment
even for failure to pay.  I agree you should make a detailed narrative
of what went on and notify the state dental board.
    As far as the money is concerned, if the dentist is a member of the
dental society in many localities there is something called peer review.
 You do give up your right to sue if you pursue this option, but this
doesn't sound like a law suit situation anyway.

Steve

Very long story somewhat short

My mom went to her dentist to take care of a painful tooth and he
determined she needed a crown.  She had dental insurance which has
since ended.  The dentist started work and put in a temp and waited for
the crown to be made.  Meanwhile there was something wrong with the
insurance which took a few months to resolve.  The dentist kept pushing
my mom to take care of the problem so he could finish the work.

Eventually, the insurance came through and sent the dentist the check
to cover the crown work.  Meanwhile, the temp my mom had fell out and
she spent months in pain and with bad breath.  Infection?  The dentist
wouldn't take care of it until the insurance company came through.

Anyhow, upon receiving the check, the dentist refused to finish the
crown.  When my mom went down to the office to schedule an appointment
to finish the crown, the dental office tried to get her to pay over the
remaining $40 she "owed" (she had already paid $290 months earlier).
When my mom said she would pay after the crown was put in, the office
told her there would be no crown since her teeth had "moved" and too
much time had passed.  They would give her back the $290 and she would
have to scram.  All this after receiving payment from the insurance
company!

My brother will go down to the office today to have a talk with them.
If this doesn't resolve the issue, I'm advising my brother (I'm three
thousand miles away) to notify the insurance company, the local and
state dental board. Is there anything else we can do?  After all this,
I sure do not want this dentist to work on my mom's teeth, is there a
way to go to another dentist and have it covered?

This dental office services mainly an immigrant clientele and I think
this is not the first they've pulled stuff like this.  They can get
away with it because many of their patients do not have the financial
and language wherewithal to fight back.