Hi @Eliza86
Airbnb take host cancelations seriously because of the impact on guests and its brand. If you aren't sure about the guest you have 24 hours (if you aren't using Instant Book) to ask the guests questions to make sure there is a good fit and if not, then you can decline the booking.
If you cancel the booking you will face financial penalties and it will show on your reviews which will put guests off from booking with you. If you go to the Airbnb Help Centre and look up host cancellations - it will give you information about this works.
Sorry I don't understand what you mean by 'his friend will put a request in, instead?'.
I would say about 50-60% of my guests don't have reviews, however I vet them by having verified photo ID, asking them why they chose my place, plans for their stay etc.