Firstly I would talk about the legal age to be able to have sex. I would talk to her about repeated enthusiastic consent. Don't buy the bullshit of we won't be doing anything because grown adults get "carried away" and the teen years are heady as hell. Any grabbing of her hand and putting it on his groin is not consent, otherwise she would put it there willingly.
Talk about contraception especially condoms that also protect from some STIs. I would also talk about period tracking, pregnancy and how she would need to test for pregnancy. Would she come to you if she thought she might be pregnant? I would talk to her about the options if she does become pregnant and the difference between medical and surgical abortion and the number of weeks pregnant for each option and how they date a pregnancy ie before you have actually ovulated.
I would talk about not sharing photos as anything under 18 is illegal and could see either of them on the sex offenders register. That if she chooses to do this that he could show it or send it to anyone. Also he could tell everyone in detail what they do together. Look, he might be absolutely lovely but he might also be a typical teen boy. I remember a boy I knew asking everyone to smell his face having just performed oral sex on his girlfriend, this was his way of proving it, he was 14 as was she. Sad, but true.
I would also tell her there are many things she can do that are not sex but feel good. Her body is not his personal playground to get satisfaction from. She is a person with feelings with wants and needs too. No means absolutely no and she can stop anything at any time.
I would invite him round for dinner, get to know him, talk to him. They stay downstairs, they are not unsupervised, she is 13 as we speak. In my child's school a girl was pregnant at 15 and the Dad was 14. It happens way more than you think. She kept the baby. Play adult games, get adult responsibility.
There is a belief that we are not mature enough to deal with the feelings and potential fall out of dating below the age of 16. That there should be dating in groups rather than 2 people on their own.