If it's a dangerous issue-std, life threatening behavior-definitely share.
If not-depends on the circumstances.
I usually let them figure it out on their own most of the time but if my red flag starts waving at me? I have to respond to it. At least try to do the right thing even if the easier thing is to turn a blind eye.
I had a weird position once...
BF, in the sharing of his past loves, revealed one of his ex's when he'd started to date her? Came to find HER previous rship included hitting and violence. (ugh!) So she was an abuse survivor.
BF and I were LDR, and then this young woman came into my circle of friends and I noticed a certain person sniffing around her skirt who I considered unethical and skanky in his "love 'em and leave 'em" ways. He'd brag on his conquests. Yuck.
I did not even like the girl -- her personality and mine are like oil and water. But I cannot STAND to see a person wander into a potentially harmful relationship to be USED. Mr Skank tried once to put the moves on ME and got really fresh and touchy. Guh!
She did not know I knew some of her back story. But I could not UNKNOW it and I could not forget my own boundaries being trespassed by him.
So I pulled her aside one day and I said straight up to her...
"Look. We are not tight friends. But as woman to woman? That guy? Be careful if you choose to go there. He loves 'em and leaves 'em and brags about his conquests. He crosses boundaries. To me that is not ethical. But I could be a nutjob for all you know. So talk to your ex -- my BF -- someone you DO know well that isn't a nutjob. For the verify on him. Other common people that know him. You are a grown up and you make your choices. But put your radar on and ask him for his full story up front and see if it matches his street cred or not. Nuff said."
Even if I did not like her personally, she did not need another UGH thing in her life.