1 The user will be notified if they're active
Unfortunately, if the person you have sent the friend request is active, in that case, the user would get the notification of your friend request. If you delete it, then also they would get to know.
2 They would not know if they are not active
If the person you accidentally sent the friend request is not active, you can quickly delete or cancel the request, and the user would not get to know about it.
3 It can be cancelled
A friend request can be cancelled by the person who sent it in case it was sent unintentionally.
4 The user won't know that you cancelled the request
The person will not be notified if you decide to cancel the friend request after accidentally sending it.
5 They might get a notification on their phone
If a person has notifications for friend requests enabled on their phones, they will get a notification that you sent them a friend request. (See Why some people are always online on Facebook?)
6 They won't find the friend request
If a person sees the notification that you sent them a friend request and you cancelled the request, they won't be able to see the request, accept or delete it.
7 The notification will disappear when you cancel it
After cancelling an accidental friend request, the notification will disappear from the user's devices. If they manage to see it before it disappears, it will disappear after they refresh the page. (See How does Facebook friends suggestion work?)