There are two situations when you want to contact a friend you do not follow.
If you have his/her username, ENS, or wallet address, you can find his/her profile by typing his/her handle in the search bar on top of the search page. Click into his/her profile then tap the chat button on the top bar and you will be brought to a private chat room with that friend.
If you don't have his/her username, ENS, or wallet address, unfortunately, we will not be able to find the user directly. You may try looking for that account in your friends' followings if there are mutual friends.
If you know which NFT he/she got, you can find the his/her username, ENS, or wallet address on the owner dentils.