In my opinion, you don't tell someone. I have played for a pretty long time, and you could phrase it in the nicest way possible, doesn't even matter that your intention is to help the player. Even with a mentor crown, it doesn't help. More often than not you will get a variation of the "you don't pay my sub" comment. Very few players actually go, okay I will try that. So, i just stopped trying to help people in dungeons (unless they specifically ask for help, which rarely happens), you are probably never going to see said player again anyway.