If they are already divorced, it is not adultery, no matter who the father sleeps with. Plus it's none of her business. She lost the right to know any of his private matters when she left him. I don't really see not telling the mother as revenge, but as an unwillingness to allow her and her JW rules to cast any of the ex-husband's private matters in a bad light.
Besides, if SHE hadn't divorced HIM for "spiritual endangerment" in the first place, she wouldn't have her hoohoo in the vice right now. Heard this same sorry story one too many times. (((Unbeliever and Father))) you don't owe her and her rules anything.
My $.02.
Odrade
**edited 'cause it sounded really dumb. HA!**