G.B.
I am a firm believer that a binky is much easier to get rid of than a finger and if she needs to suck then she needs to suck.
Next time the friend says something about it stop...just stop what ever you are doing and look at her and say "Stop". Ashe her "Do you realize you are constantly nagging me, arguing about this, and I am done with it. We are different moms and I love you, you're my BFF but this has to stop. I don't want to be around you anymore if you can't just stop.".
This should get the message across and she may have good valid reasons for keeping on you about this. Maybe she has buck teeth that needed years of braces from sucking a binky when she was little. Who knows better that you"? But this will let her know you heard her and chose not to follow her suggestions.