This is a difficult situation, weight the pros and cons carefully. Only you can decide if you shoudl sue or not. To some it would not be worth it, to others it might be.
Have you contacted a lawyer yet? Just for a consultation? A very good person to contact would be Beth Suffian-poor Beth, she must receive 100 phone calls a day from all of us...(she's an awesome person, very smart and always helpful) She is a lawyer in Texas, she has Cf, and she specializes in Cf related cases. I'm not saying she'll take the case, but she'll probably be willing to talk to you over the phone, get the facts, and advise you as to whether or not you really have a case. She'd also know if there are guidelines (Cf foundation or otherwise) as to how low a persons PFTs should dip before treatment.
Regardless, as a layman, I would think that even if there aren't written guidelines, it does violate a MAJOR ethical code, and the bottom line is sadly, a person with Cf could die under that doc's care. There should be consequences.
I was in a situation once, and I can't go into detail, but it a teeny bit similar. I person harmed me and I wanted to take legal action. I knew I probably wouldn't receive anything that would compensate in any way for what I lost. But in the end the reason I did it was for other people that we felt should be protected from this person. BUT like I said, everyone's different. You may not be in the same position. If you chose not to do anything, I wouldn't think that was wrong either.
I think you'll have a hard time finding a doctor to testify whether in person or by written letter in your behalf. A shame, but all too common. Maybe just see what you can do, call Beth, or another lawyer just to talk. And you were wise to not name the person here-I wouldn't give too much away if you intend to sue!!