Your girlfriend is only 17, did you get her parents consent to have sex with her? She’s a minor I do beleive,if the age of consent is still 18. Perhaps you should wait tell she is 18 before you have sex again with her. If you did not get her parents consent, at least that’s the way I think it works. They can put you in jail for statutory rape. Even if she gave her consent.
Fuck that, hes 18, everyone does it, if he trusts her, then I see no problem. Here in the UK its 16, I feel thats more right.