My first bit of advice - tread very carefully here!
In my line of work confidentiality breaches are taken very, very seriously. (As in instant termination and loss of professional licensing! In addition to multi-million dollar lawsuits and potentially federal investigation if the employer was found to be lax about enforcing regulations.)
That being said, not infrequently I find myself in situations similar to what you describe (with the caveat that my colleagues and I cross-cover so it is conceivable that the conversation could have happened under a "need-to-know" basis).
So, you can't tell the potential victim that you saw the potential abuser at work. If you meet the potential abuser socially you cannot publicly acknowledge that you have seen them at work (unless they acknowledge that publicly first). Now, they may very well assume that you have spoken to your friend (people often assume that other people would do what they would do) and may accidentally out themselves asking their partner what you have told them. If you see them again at work, they will "know that you know" that they work with someone there - and, again, may out themselves by their reaction. (You can acknowledge that you recognize them the next time you see them at work because you have met them socially - because there is no "expectation of privacy" relating to a social situation.)
As far as your friend goes - you can watch very carefully for "red flags" that your friend tells you (i.e. things that you learn outside of the info from your colleague) and voice your concerns with regard to those behaviors. "You say that he always wants to know exactly where you are and who you are with so that he knows that you are 'safe'. I have to tell you that I find that more concerning than thoughtful - as I often hear of such behavior as part of a pattern of control as opposed to a genuine concern for a partner's wellbeing."
You can provide relevant "general advise" without disclosing what you know about the partner as an individual. For instance, it is always a good idea for a person to have access to money and their private papers that a partner does not have access to "just in case". You can express concern about introducing children/dependents to new dating partners "too quickly", etc. (Are you a mandated reporter? Because that can come into play if you learn of any abuse of minors/dependents even if you don't learn of the abuse through work.)
My friends and family are well aware of what I do and that there is some overlap in my social and professional circles. Occasionally, I will get asked if I "know" something - my response is always "You know that I couldn't tell you even if I did." (Whether I do or not.)
(NOTE: you could also have done all of this withOUT confirming with your colleague and therefore creating a confidentiality breach)
The way the potential abuser has been recognized is through a FB profile. So at this time, the abuser likely does not know that their date's friend is somewhat known to them.
I think we will all be giving general advice as you stated. I just hope that we aren't too much with it: "WHAT?! HE ASKED YOU THE TIME?! WHAT TYPE OF ANIMAL IS HE?!"