Question: How do I break the news to my friend that I have been seeing her (recent) ex-boyfriend? I'm prepared to lose the friendship, but I want to minimize how hurt she gets, if possible. I guess I'm thinking specifically of altering the timeline, so there's more distance between when they broke up and when we started dating.
Answer: The first three letters of "timeline"? TMI.
Oh, close enough.
To be clear, you and he have already taken the action that hurts your friend, so there's no altering history to cover your backsides. There's only consideration of how much of the truth she needs, for her sake, and supplying it accordingly.
Without the details, I can only guess at that magic quantity. However, most in her position need only the fact of your dating. That, and respect; no fibbing.
What your friendship needs, meanwhile, is for her to hear it first from you (though that might spell the end regardless).
Combined, those dictate a simple statement, soon: "Ex and I are seeing each other. I wanted you to hear it from me first."
If she asks when it started, please use this truth instead: "Any time is too close to your breakup, isn't it?" Meaning, admit fault out of integrity and skip the details out of compassion.
If she insists, then she assumes the risk of knowing all - like signing a pain waiver. Honor that with the truth.