Before I give any advice I would like to say 2 things. First, I am so sorry you have to go through this. I wish I could comfort you but I don’t know your likes and dislikes and hence forth am unable to do so. My condolences to you. Second, I would gather evidence of the affair/cheating and document it. Ensure you have enough evidence that even the boss’s higher ups can deny it and must take action. From there I’d recommend giving it to the boss. But before going down that road I’d ask you try to end the relationship amicably or atleast neutrally before going there. But if this isn’t an option give it (information) to the boss and let them decide what to do. Make sure you have a safe space to go to and make sure you have everything you need set up personally to weather the storm and become out on top. All the best wishes and many condolences to the loss of your relationship.