I am going to be blunt here: If I was R's friend, I would tell him to stay away from you.

It is not just one of your actions that has added fuel to the fire here, it is many of them.

I would be devastated to hear that my ex hooked up with someone else immediately after a break-up (you technically did nothing wrong as you were broken up but the short time frame would be an issue for most people), But to hook up with S on R's birthday and then tell S about the inpatient item -- that is just too much.

You can yell and scream at S all you want but you should not have told S -- period. And that is all on you.

Please let R heal and that can only happen if you are not part of his life right now.