Ronnie and Mary's Fling

Does anyone actually buy that it actually happened? To me the weird part wasn't it's plausibility but how Ronnie aired the news. Why was that bit of information something she "just had to tell" Robin? Why was it so pressing of a matter to get of her chest? And after she said it, she was just like, "well, it's said now I'm just going to stare lazily out the window until we reach our destination." Seriously, wtf. I'm having trouble believing that Ronnie didn't just make it up to try and isolate Mary from one of the only friends in the group she has left.