I mean, they tell me something, say, they're going on about their partner being a bit of a tosser... I might tell DP. I won't run home and blab, but I might say something. It's no big deal. I might just say, "I can't believe she puts up with his nonsense... she really deserves better," as I vent to him about it or whatever.
If it was that she was, as a said in the OP, sexually assaulted, I wouldn't bring it up to ANYONE that might know her. If I really needed to talk about it and wasn't seeing a psych, I'd probably start an anon thread on here with only very generic info. Just because I know that everyone needs to unwind. I think I'd be okay with someone doing that with my problems too - just not telling people I'd have to look in the fact at some point, who would know all about it, but who wouldn't say anything to me about it. That would weird me out.
I tend to expect that anyone who I tell stuff to will tell their partner, and I don't have anything so secret that I ask them to keep it to themselves... but if it was something very very personal and touchy, I would request that they don't share it with their partner, or anyone else that knows me (even if they don't use my name). I'd rather they talk to people who don't me.
I don't tell DP everything anyway. I mean, I don't go, "Okay, so I woke at 7.14am, and went to pee but someone had forgotten to replace the toilet paper and there was none in the roll-holder, so I had to do some weird hobble-walk and hope not to drip anything on the floor... and then after I grabbed a roll, I hobbled back and flushed... for lunch I had this, that and the other, but I burnt it a bit so was kind of cranky and so chucked half of it out... and then later when I was on BH some fool wrote something that made me wants to smash the computer (over her head... lol), but instead I said..." etc etc. You get my point. Omitting stuff is kind of what we do anyway... I don't find out every little detail of DP's day and he doesn't know mine because it's impossible to do... so I don't feel that keeping secrets that have nothing to do with him is a betrayal of any sort.
I'd be cool with him not telling me that his friend might have gotten an STI from someone, or had just found out his kid isn't really his, or whatever other personal matter he shared with DP. It's not my business...