The oncologist wasn't lying, he was expressing his opinion. Most likely he wanted to help, but was going about it the wrong way. Your Mom made her choice and it is entitled to be respected. Giving advice in a heavy-handed way is not malpractice. Since your Mom did not take the advice, no harm done.
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For a medical malpracice claim one needs to have damages. Your mom did not go with his recommendation and so suffered no damage a result of anything the oncologist recommended. But even if she did and got the chemo and suffered more damage as a result, it would be a very difficult case to prove. The oncologist is entitled to his opinion and to recommed a common treatment for cancer.
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