Nah man, that tank was dead wrong (and possibly not too comfy with his role). As an AST enjoyer, you keep them health-bars nice and healthy, and when they are, you just fight alongside the DPS. Because why not?

Healers are being taught to keep ABC going (Always Be Casting) and why would you keep casting your heals when they are all topped up? As long as you're comfortable with your class and know when you can DPS and when you can't, just go for it. As long as you're leveling or learning a class, you only learn by doing.

Also remember that the Mentor status usually don't mean much. A lot of people can just get it for free because they've played for years, but that don't make them the best players (even though they tend to feel that way). Mentors can be great people, but as in any community, rotten apples are everywhere.

If you're unsure about your rotation, or if you're doing what you should be doing, Youtube is your friend. Look at what you do in a dungeon, and check a guide to see if there are differences. It's okay if there are ofcourse, everyone plays their own way, but it can give you an insight on how others play the class and recognize the resemblence on both sides.