J.L.
I have a sister somewhat like this and I have had to confront her. I tell her very pointedly that I will discipline my own child, and when she does the tattle stuff I just smile and say-"Oh kids are so funny" or something equally benign that lets her know I am not going to jump up and reprimand my child every time she sneezes without covering her nose.
But my sister is also very young and inexperienced and has always seemed to be easily irritated by everything my daughter (the oldest ) does-so it was uncomfortable when I finally lost my temper with her over some minor incident because I had been bottling stuff up. So-if you are friends and if you want to keep the relationship be candid with your friend. Just ask her' "What are you so worried about? Do you really think that doing...(whatever)... is so terrible?" And tell her that you wish you could help her feel more comfortable at your house, because it seems like the kids are stressing her out so much-and ask her what would help her relax.
I believe that if the friendship is real it should be strong enough to handle some honesty. Even ask her "Does it bother you that I don't seem as worried as you are about how the children are playing?" then explain to her why you aren't so you can both talk about it.