Wow, I am shocked that nj even told you,
Most nj's would threaten and convince the child not to tell you and keep it quiet.
I agree with Trevor, nj is trying to most likely divert the attention to the neighbors child here. There is more to this story.
Timing - Wait till son is with you and talk to him, buy him a soap bubble toy, play with him, divert his attention first and then slowly reach out to him. He will be on guard as nj most likely will have done all sorts of coaching.
When did it happen and when did she tell you?
I would ask her in an email when it happened and ask where she was and her bf in a follow up email, before asking her to report it all to the police.
You have to come out as not serious to her and like you don't care. Yeah, don't forget to ask other distracting questions like, what son ate that day, how is the dog etc to throw nj off in the same email.
After you email nj to log police report follow up with an email asking for confirmation if she has done so.
Then you go in and do as Tom and others have suggested.
“A woman worries about the future until she gets a husband, while a man never worries about the future until he gets a wife.”