If I notice DPS seems to be lacking I will try to keep an eye on the rotation they are using, if i notice any flaws I will politely say something along the lines of "why don't you try this out, maybe it will work better for you, I've noticed it helps me out a lot". Never say something that will make them feel flawed, and if possible maybe say it away from other people in private chat, keeps the person from feeling embarrassed.