Then I have to agree with you. If the person think he's doing good but he's obviously fail. You can't do much but I,ve never had to deal with it in DF. People usually take my advices because they want to improve. When i face that situation, I just leave the duty and craft for 30 minutes.

There's also two way to give advices.

The first way: "Stop being a noob and save your CDs to burn the snakes. For God sake."
The second way: "In this situation, you have to save your CDs for the snakes, they need to die asap or the tanks and melees will die from the debuff."

I can guarantee you the person will most likely listen to the second way more than the first way. There's more details and you haven't insult him.