Your friend is perpetuating an old quilting myth. Today's soft poly threads will NOT tear your cotton fabric over time.
Choice of thread should be entirely based on the look you are going for. If you like poly, use it. If you like the look of cotton use that. I do know some LAQ hate using cotton threads because of all the lint in produces in the bobbin case. Lint can get caught up in the bobbin case spring and cause the tension to go bad. A nice high quality cotton will not cause as much of a problem as some lower ends.
Anyway, by all means go to your LQS for long arming if you like the look of what she does and maybe take this opportunity to help educate your misinformed friend. Here is a text version of Superior's Q&A regarding the subject if you don't or can't access the You Tube.
http://www.superiorthreads.com/educa...ar-the-fabric/