What happens if a recording artist gets bad advice from an entertainment lawyer?
What you are asking is really a smart question.
You see, one of the major misconceptions many people have is thinking lawyers are perfect and know everything there is to know in their particular field of practice.
And that they won't miss any cleverly hidden traps and clauses written by lawyers more intelligent than them.
There are several incompetent lawyers out there — an artist has by default an obligation to do research about a potential legal representative's track record before hiring them to look out for their interests.
Nonetheless, a way forward in such a scenario is to sue them as
@Scorpio said.
But that won't necessarily change the prior terms and conditions in an agreement since a record label or a production company will definitely flat out refuse to re-negotiate once a deal is struck.
They will tell an artist to drop a couple of albums first then maybe we can come back to the table to address any issues of contract re-negotiations.