We have 2 girls..ages 29 and 21
I allowed them to go to sleepovers like the one you describe. I trusted them to act responsibly and they did. I have had hundreds of compliments on how my girls carried themselves.
Both graduated high school, my 21 yr old did so with honors. And now is the proud Mom of Raiden and Bella. My other daughter is an Upholsterer/ Seamstress with a contract with Disneyland in SO. Cal doing their furniture and drapes.
When my youngest was 15, her bf stayed at our house overnight several times. He slept on the sofa and he knew my husband, he stayed put....LOL
It's a matter of trust.