What to actually say
Open with this, more or less word for word: Can I say something that might land badly? I'd rather say it than keep sitting on it.
I'm not calling him a bad person. I'm telling you what I saw, and it worried me.
Separates the verdict on his character from the specific behavior, which is the only part you actually witnessed and the only part you have standing to speak to.
When he said that in front of everyone and you laughed it off, I don't think that was actually funny to you.
Names the exact moment instead of a pattern, and gently questions the cover story she gave herself in real time, which is easier to hear than a general accusation.
I could be wrong about what it means. But I'd rather say it and be wrong than watch and say nothing.
Admitting uncertainty up front takes away her easiest counterattack, which is telling you that you don't know everything.
I'm not asking you to leave him. I'm asking you to notice it happening the next time it happens.
Shrinks the ask from a life decision to a moment of attention, which is something she can actually agree to without feeling cornered.
How it usually goes
A version of the conversation with the push-back left in, because that is the part you need to have already heard once.
What not to say
You deserve better than this.
It sounds like a verdict on her whole life, and she'll spend her energy defending the life instead of hearing you.
He's abusive.
A label that big forces her to defend the entire relationship on the spot instead of considering the one moment you actually saw.
I've never liked him.
It reframes your whole friendship as something you've been lying about, which costs you the trust you need for this exact conversation.
If they defend him immediately, before they've really heard you
Don't argue the defense — you'll end up debating whether he's a good guy overall, which you can't win and don't need to. Drop that thread and go back to the one thing you saw. Say: I'm not going to debate whether he's a good guy. I just want you to remember I said this, if it happens again. Then actually stop. Repeating yourself now reads as pressure, not concern, and she needs to be the one who brings it back up later, not you.