How does Xbox friend request work cross platform?

On the 360, in order to be friends with someone they must accept your friend request. This isn't the case for Xbox One friends - you can add them without them adding you.

The Xbox Support site indicates that you can add Xbox 360 friends while on the Xbox One:

Keep in mind that friends you added to Xbox 360 will show up as both friends and followers on Xbox One.

However, a friend that has been added on Xbox One will not automatically show up on your Xbox 360 list. If you plan to play with them on your Xbox 360 as well, you will have to send a friend request through your Xbox 360 console. Once they accept, you can see them on both of your consoles.

You are not sending a friend request from one console to another. You are sending a player a friend request on Xbox Live.

Each console (and PC generation for that matter) implements the Live features differently. to answer your question, if you send a friend request on the 360, the other user will see a pending request on the 360 and a new follower on the One. They will not see a pending friend request.

I have found a way simply add the xbox 360 smart glass app on a phone or tablet, then sign in as the receiving friend, you will find the pending request. Confirm the request wait a few mins and that's it. It works but is frustrating surely Microsoft could have found a better way bit of a work around.

If you’re on your Xbox one console, start a backwards compatible Xbox 360 game. Once the game has loaded, pressing both the View and Menu buttons (perhaps more commonly known as Start and Select) on the Xbox One controller will open up the Xbox 360 guide when you're inside a backwards compatible game. Here you can go to friends and see friend requests from your Xbox 360 friend.