comfort, darling, you did absolutely nothing wrong. Your DP has let you down somehow on this.
Give it a few days, it'll all calm down eventually. At least the truth is out and it's better for all really.
Important piece of advice? Hold your position, you have right and reason on your side. It is NOT your responsibity to mind-read and cover up the indiscretions of others.
The view from the moral high ground is fabulous... If anyone says anything to you, tell them FFS, sort yourselves out, stop acting like kids. And refuse to discuss it further.
When it is calmer, take your DP to one side and explain to him what has happened, i.e that someone has cheated and he has covered up for them, and that is WRONG. Say to him that if he ever betrays you again, then it'll be a much more serious situation than it is right now.
Don't take this infantile behaviour, tbh, it's beneath you.
best of luck, and good for you to want to be there for your friend....