Personally, I don't care if it were my best friend from birth who asked me not to tell, if it were concerning my child I would confront the situation and just ask outright. (edited to add, I realize this is your stepchild but still from the dad's POV)
He needs to know where his baby is at all times, you just never know what may happen. Probably there would be no cause for concern, he is that child's dad and he needs to know everything. Hopefully your friend's info was inaccurate but obviously you need to get to the bottom of this or you cannot let that child go.