You can ask a friend for a sticker from finished Events at any time, with the exception of prize stickers.
You can only give away duplicate Stickers. These can be found in the gift menu of the G5 Friends service, next to the collection items (Friends > Friend A > Gift > Send a collection item). Tap/click on a sticker to give it to your friend.
You can also give a friend a sticker by selecting it from their wish list. The sticker you give them will be removed from your set.
You can only request stickers that you have not yet received during one of the finished events. Tap/click on the empty sticker space on a page in the Moments section of the Diary.
The sticker description window will open. Press the Request button and the sticker will be added to your wish list, which your friends can see in G5 Friends. If one of them has a duplicate, they can give it to you.