Just snuggle her.
He should become interested soon and it may end up being very difficult to keep them separate. It is best to take one of them to someone elses house so there are no accidents. Otherwise they will have to stay separated by closed doors but one slip up and she may end up pregnant which would not be healthy at this age. Make sure you don't just crate one of them because they can still tie.
If a female is spayed before her first heat her chances of developing mammary cancer are less than .5%. If she is spayed after her first heat and before her second her chances go up to 8% and if she is spayed anytime after her second heat it goes up to 26%.