I'm sorry your trip was ruined - but we all have choices every day - and one of those choices it to move on and make the best of whatever situation we're in.
Many points i would like to address, but I'll just stick with this one.
I hate to break it to you - but if she is allowing ONE dive from ONE dive op at ONE resort to paint all of Cozumel and all of the dive shops here with the same brush, after you yourself have had 3 previous SUCCESSFUL trips here - then you probably have much bigger issues to deal with than this one messed up day that you made the choice to be angry over and allow to ruin your trip.
I will go ahead and add - that unless you were chartering a private boat - the crew on the boat has to take everyone into consideration - just because YOU wanted to stay shallow doesn't mean they should hold the whole group back. If your needs to stay shallow were that important, and your friend is that new of a diver and nervous to begin with - YOU should have hired a private dive master for her at minimum. I'm sorry, but with less than 100 dives yourself, you are still considered a novice diver as well by most standards.
I am not making excuses for the dive op - BUT - You were diving with a large resort op, not a small boutique shop that offers more personalized level of service. No, I would not dive with that op before or after your report - but if I did find myself diving with a large run of the mill resort op catering to all levels desires, etc. I would keep my expectations and personal responsibility in line with that. I think your expectations were too high - you wanted caviar and paid for anchovies - and that's what you got!
Good luck, best wishes and I hope that you will both be back to Cozumel with an open mind and willingness to give the island and another op and resort a chance to show you some excellent and safe diving.
And I do mean this with the best of intentions and simple honesty so you can maybe make more informed choices in dive ops next time