Well, the R-rated pictures would have made her BF mad & upset, I should think, even if she was the one who sent them. I think you are right, being flirty to a point if you have no intentions of sleeping with someone and just being friends and only joking about it to a point. If she sent the pics unsolicited, IMO she was the one who did stuff that would break off her friendship with you and cause trouble with her BF if he found out, especially the pics.

I am sorry about the loss of your friendship though