Just because he's not her biological father does that free him from any culpability here where the children are concerned?
Yes, it does. The SD HAS a father, and she is HIS responsibility, just like awww's SON is awww's responsibility.
LOOK, we see BH's come here everyday with a whole list of EXCUSES for why they are doing NOTHING to protect themselves or their children due to being paralyzed by fear of their WW's wrath, and there are usually a few well-meaning emotional do-gooders who will support and encourage their INACTION.
HOWEVER, the successful BH's are the ones who understand the things they can actually control and take immediate and definitive ACTION to protect and defend those things.
Your advice only ENABLES and REINFORCES awww's inaction by suggesting that he has a responsibility for something that is OUT OF HIS CONTROL.
In situations like this, I go back to the source of "my revelation" (hence my username) when I was struggling with the potential of my W's infidelity (prior to confirmation and confession) and looking for information, when I came across that old Serenity Prayer:
God grant me the serenity
to accept the things I cannot change;
courage to change the things I can;
and wisdom to know the difference. These four simple lines gave me the clarity of thought to concentrate my efforts where they could actually do some good, rather than tilting at windmills in a futile attempt to change things that were outside of my control.
We all recognize that awww's mind is pulling him in a thousand different directions. His whole world has just been turned upside down, and he's frantically searching for answers to questions that have no real answers.
What he needs more than anything is the wisdom to focus his efforts where they can do the most good to help his situation, and advising him to risk EVERYTHING on something that is outside of his control is not very wise at all.