Well I am about the same age as that adult and the idea of having a relationship with a 16yo makes me want to vomit.
That said, if you are worried about pregnancy, I don't see how a woman is accidentally going to get your daughter pregnant.
But anyway, 16yo needs to be reasoned with and come to the understanding that it is not appropriate to be in such an unbalanced relationship. All the power lies with the older one, your dd isn't old enough to drive or drink and might not have even left school/education! She's a child and even though she might not feel or look like one, she needs to realise that person has another lifetime's experience compared to her.