I don't think it is appropriate for a dental assistant to be scaling teeth.  Going to the dentist is a scary experience for most, and it definitely is for me.  Even the best dental hygenist and/or dentist can be a little rough, but I don't think a dental assistant should be allowed to be involved with scaling teeth.  If he/she wants to participate, it is better to continue their education to be qualified to do so, and that education should bring her to the level of a dental hygenist.  Thank you.