OK.. It depends..
If I was creating a multiplayer game, I would probably have an own server to handle with this kind of thing.
A backend server with API can handle that. So on Construct you will be using ajax requests to know if your friends are online or not, asking it to the server.
To "Save Friends" you can use local storage, it will just save the name/id of the friend, so, on construct you can do like:
Every 5 seconds
For each (array value with friends id) ajax request
- set status icon to green
- set status icon to red