If it works for you, is a good quality thread and won't destroy your work over time, use it! I use mostly 100% cotton thread when piecing and sometimes I use a poly thread (like Superior) when appliquing if I can't find a cotton thread that matches the color of the applique.
If you hate the thread, don't bother with it! Some machines do get picky with certain types of threads and no matter what anyone else says, it won't make the machine work better with a thread it doesn't like. You know your machine and what works for you. Politely thank your friend, tell her you will consider her advice, then just ignore it. She feels better and it keeps any problems from occurring. ;)
It's true that my machine didn't do well with the poly thread but then neither did I. :) I'm happier with my machine when I'm using the cotton thread.
As to my friend, I'm know she thinks hers is the only opinion that matters but I will ignore that as I have done with her comments concerning the cotton thread. I chalk that up to too much poly thread in her possession to just chuck and go buy cotton to replace it with. :)