The mother didn’t know his age, name or who he was! So being 17 had nothing to do with the mother killing him.
When I was 17 I took my 14 year old girlfriend to a late night movie. She snuck out of the house, we went to the movies, and that’s it - I swear, and sneaked back to her house. If her dad had caught me driving her from or to their house, I bet he would have shot me even if he knew exactly who I was!
My point was that someone seemed surprised that 17 year olds could have girlfriends two or three year their juniors.