She could also just be in complete denial where she doesn't even want to acknowledge it herself.
If she's gone 30+ years without him in her life, and she thought it was all behind her, it's probably pretty jarring to have obnoxious strangers suddenly bombarding you about this shameful thing from your past seemingly out of the blue.
Also, she might not have even told her current husband she had a child in her past and she could be trying to keep it a secret from him. That could explain the vague responses she's made for plausible deniability (ie, feigning confusion, "I'm his step-mother" referring to her stepson Josh, or "I think the last name was Saunders" to appear clueless). She can't outright say "I'm not Josh's mom," because even that shows a level of familiarity with the situation.
Something like this could cause a lot of upset in a marriage, especially if she previously lied/mislead about not having children.