Hi Rivergal , we had 5 teenage daughters at home at one point,and i remember well the issues!
I would not allow her to stay over at her new boyfriends house, have you met him yourselves?
To avoid coming out with a straight no! suggest you meet him, and get him to come over to your house for a meal or something, and you get to know him better for your peace of mind.
At 16 they do think they know it all, but we still have a duty to protect them too.
Is this the first time she wants to stay at a boyfrieds. At least she did ask. Say no but explain why, dont fall for any stories like "well all my friends can stay with their boyfriends"