Boyfriends daughter

Hi. So recently my boyfriend’s daughter came out to me as bisexual. She isn’t comfortable telling her dad that yet so I’ve kept it to myself.

However .. she had her best friend stay the night a few weekends ago and she recently told me they kissed during the sleepover. They’re both 12.

I’m not sure what I’m supposed to do with this. Telling him would be outting her but I also don’t think he’d be okay with his 12 year old daughter having a sleepover and kissing during it.

What should I do ?