If you do not want to change this in the computer, then download the app. See where your friend is (have to ask them first). See if there is a seat empty near by. CHOOSE IT! We often juggle our seats a few times until we get what we want. If you do it early enough, before everybody else beat you to it, it is easy. So either YOU have to choose near your friend, or you pick a seat with empties next to it, and THEY course to move near you. You should be together when you do this - or at least on the phone. If you call the airline, they do not have BOTH of you in the other unless you are together - so using your apps might be easiest. You will learn how to do this for the future!
Have flown maybe 100 times on Delta - airways pick our own seats when we book the tickets on the computer.