I agree with others who have said that women perceive things differently from men, particularly regarding relationships and other women (you know, the Mars/Venus thing.)
That being said, women know that a quick way for a man to get into another woman's heart is through her kids (showing care for them in any way, nurturing them, etc.). This "feeling" is pretty primal, based on sociological constructs created by our (evolutionary) biology that bestows advantages on this type of "behavior".
Your wife may be perceiving that this other woman is giving you the "status" of a father figure in her child's life and this is threatening to her on a very base level, rational or not.
Question: Since both of you drive her kid around or interact with her kid (i think that's what you said or implied), did she send your wife a text saying "Happy Mother's Day" last month? Just askin' as more of a rhetorical question.