Contact the guest through BNB. Mention that they have the booking for one person but understand from their friend they will be bringing four. If this is the case, you can alter the booking to accommodate four with the increased price.
Tell the guest that they will need to accept this, before staying in your home.
If they say only one person is staying - confirm with them that only one person will be allowed to enter and stay in your place.
If you don't want to have them in your house because they have lied or you can't accommodate four then contact BNB once they have confirmed numbers and ask them to cancel the booking.