The OW is an adult in her 30s. She is old enough and experienced enough to evaluate potential partners for suitability.
If she does not know your WH is married---because he's willfully lied--that's something I would let her know.
But I would not devote even a moment of concern to what she might or might not know about his character; it's HER responsibility to assess that.
Often, women who have affairs with married men don't want to know the truth. The "couple" constructs a fiction that fuels their infidelity.
Anything you tell her is apt to be used as "evidence" to support their self-deluding stance, further fueling the affair.
Beyond, "He's married" (if she does not know), don't waste your breath.
She's not your responsibility.
OTOH, if she's married or in a relationship, I'd let her BS know. He deserves the opportunity to make informed decisions about his life, including the decision to be tested for STDs. And having another set of eyes can be useful.
Millions of hugs to you.