How do you bring an adult son out of denial about the clever, abusive woman that lives with his dad? He doesn't see the red flags.

Please elaborate. Otherwise I will have to guess that perhaps he is more loving and tolerant than you had previously guessed.

She could be like the professional scammer who moved in with the dad of my brother's wife, a holy terror. But without more info, we don't know

Squeezing such a complicated question into the title of a question entirely makes it literally impossible to answer.

You do not give any info about the situation.
If the "clever abusive woman that lives with his dad" is the dads girlfriend/companion and you are divorced from your adult son's father you have no skin in the game so to speak. So really you have no say in this.
If the "clever abusive woman that lives with his dad" is your ADULT son's girlfriend /companion and the girl is residing with them then again you have no skin in the game and your son is an adult so you really have no say.

If you want to paint a clearer picture that might help.

Why is this any of your business and what would you accomplish by poking a hole in adult son's denial? There must be a lot more to the situation.