13 16 year old dating
A 16/17 year old boy is at a very different place than a 13 year old girl, emotionally, physically and sexually.
I actually think it is illegal for a 17 year old to date a 13 year old, I would talk to his parents first and ask them if they know their son is going out with a 13 year old and if they are know talk to your daughter school councilor or even go into a police station and get some info.
It is not good for her, definitely not a good experience to start your sex / romantic life. Imo it should never have been allowed in the first place.
Show her this is not healthy, let him know this is not Ok, and do not fear him. His decisions are his, if he proves unstable then better you're distanced from him, if he threatens empty threats you've learnt about him and again, teach her these are qualities to avoid in a man, it's not a healthy connection and you should recognise and break away asap. I'm sorry but first if anything has happened sexually it's illegal, secondly there is a massive developmental difference from a 13 year old girl and 17 year old young man who will be 18 within the next 12 months. My partner's just turned 15 yo daughter was dating a 21 yo guy. This was me similar rules everything we still managed to have sex alot.
I would keep her away from him as much as possible.
Don't be afraid to make her upset, you know her interests better then her, and she will thank you one day.