They will investigate, but all the offender has to do is say "lawyer" and the case is dead in the water. Unless there's someone else who was witness to some aspect of the assault you just have word on word.

Neat part your family friend left out is even if police wanted to charge him for it, prosecutor could decline to prosecute since the conviction odds are low and he wants to maintain a high rate.

It varies by area, but you could legitimately stab someone with a knife in the back in my city and it only be considered a misdemeanor because our prosecutor though juries wouldn't accept it as a felon. So investigators were forced to do misdemeanor charges or throw out the case