Sending data from one user to another through Firebase Android

So I have an Android app that is a workout tracker. The data consists of exercises/rests. I want to be able to send that data to another user, and have that other user have the option to either accept or decline the data. So, when one user sends the workout, the other user will receive some type of notification that will allow that user to accept or decline. I also want to be able to do this for a friend request scenario. I have looked into FCM, but it seems that I will need a server for this. So Cloud Functions seems to be the better alternative, however, I am extremely confused as to how it works, even after reading the documentation. Is there a way to accomplish what I am looking for, natively in Android Studio? I was thinking of attaching a listener to a node, and once data is added, display that notification. The user that is sending the data, will be sending it to that node. Since each user is required to have a username, I would know the destination for the data. Would this work, or does it pose possible dilemmas?