I don't know. You seem to indicate that the dentist made a mistake for not diagnosing a tumor. Is this what the dentist would normally be expected to do? Diagnose a tumor? If so, then I would guess that he would be responsible. But suppose that the doctor had been seen first and he just sent you friend to a dentist for a tooth extraction. suppose that your friend had then later suffered serious complications or had died because the doctor incompetently diagnosed a bad tooth instead of the tumor (which he should have been expected to diagnose). So ... should the Doctor be held responsible? Let's face it, it's not an exact science.
See the point?
As for mistakes, a young woman backed into my truck a few years ago. I had to sue and then have both her and her mothers drivers liscense suspended in order to collect for the damages to my truck. Backing in to my truck was a mistake. Having no insurance was a mistake. I suffered damages to my truck. In my circumstance, would you have just said "oh, it was just a mistake" and paid for the damage yourself?
In my business I carry a great deal of insurance to protect my customers form any 'mistake', even 'Honest mistakes' I might make. Are you suggesting that I shouldn't be held responsible for my mistakes? Mistakes are what we carry insurance for, Either my own general business liability or a Doctors malpractice insurance. Solutions for the problem lie somewhere else than relieving doctors of liability for malpractice. A precise legal definition of what constitutes malpractice would probably help, as it is often a judge or an uninformed jury that makes this determination.