oooh tricky one, yes, for I am a big coward and hate to cause stuff....
I think obviously this provider is NOT in it for the children, that is clear. I wonder what woudl have happened if you werent there...did she just assume that you were 'looking after' her, and that is why she didnt even glance over... I think next time you see her at the park, and she looks as if she is engrossed with socializing, bring Suzie over to her and say 'maybe XXXXX will help you up the ladder thsi time, we dont want you to go up by yourself...' I have to go play with Billy and Tommy....
and also next time you run into Susies mom, tell her you were with them at the park and had to help her up the ladder, off the swing, she fell over etc etc etc, hopefully the mom will ask where the provider was and that will give u the chance to say "oh, she was there, I dont think she saw any of that becasue she was busy talking with somebody....She hardly noticed...Luckly I was there to help little Susie..."
I knwo waht you mean , you dont want to be responsible for anything that coudl happen in the mean time, like a fall at the house, and then you would feel as though you should have reported her.
But lets give her the benefit of the doubt that it was a one-off thing and she truly thought you were really close with Susie and so she didnt want to interfere with you two 'playing'.
Good Luck!