Some very valid points the two posters above have pointed out.

I am guessing this person makes her feel safe. It is therapeutic to relate one's story to an accepting ear. But recognising when it's appropriate to do so comes with both age and experience. I'm making an assumption the friend involved is younger in age. Is this the case.

It is one thing to express, "I'm a little concerned" and another to say "I don't think you should."

But I actually have another notion come to mind. Why is it that you know so much about this relationship? I would distance myself from being involved.