Your question will likely be one of those timeless philosophical ones that will never have a universal answer. Not that I am a total relativist, because I'm not, but because I DO believe that these situation do fall under that relative headbar.
There are those (and they will undoubtedly speak here) that believe in taking what "they" feel is the 'high road' and avoid any participation whatsoever with a situation where things could go badly. They feel some ethical obligation to not be a contributor (to an unknown).
If only life were that simple.................
Somewhere along the line we learn that we cannot assume responsibility for 'others' behavior. And that has to even include what we might feel is risky behavior. And that is what we are faced with. RISK. An outknown outcome. But factually, it's just as incorrect to assume our participation will decide this outcome. Things will generally be what they may be and we, as individuals, are not omnipotent. When a path (or relationship) begins to diverg, there becomes many possible outcomes, most of which are totally indifferent to our involvement.
The situation you describe fits that.
The result (if discovered) could turn to an ugly, drama filled mess affecting many.
It's also possible that it could be the springboard to launching several relationships to a higher plateau.
You are NOT the decider on what path this will take. You are nothing more than one factor in a complex equation.
The ego wants to convince us otherwise, of course.
So it seems clear (right?) that this really is about one's philosophy. And that is a very personal thing - to each of the people involved. Whatever path you choose, it has to be one that feels right with your personal philosophy and beliefs.
You are doing your due diligence by questioning. Yourself - and everyone else. Be honest about the answers.
That's about all you can do...........