Have the "safe sex" talk with her and her boyfriend (invite him over for dinner to have the talk). Remove any shame or embarrassment and make the conversation about sex anecdotal instead of accusatory. Since she gave her boyfriend oral sex, they'll probably have sex next (if they already haven't). She's 18, so she's not a child. But since she lives at home, she needs some reminder about boundaries (i.e. they can't do the oral sex/sex in your home) without alienating them both, that is.