Is there a nice way to tell my friends about this?

I think you need to give him advice not to speak loudly, I hope he will not mind & you don't undermine him by doing this. It will help him to go further in his life. I think to suggest someone about his wrong behavior & habit, is not telling him that He is not good. If he is not good then he don't teach anymore. I think just a little bit advice will definitely help him to get rid of this problem..