You know, I wrote something about this and it's. . . somewhere.
Anyway, in the story, the PC cheated on Ano with Saemon Havarian while sans soul. I based his reaction on the assumption that he had grown somewhat during the game. Of course, he was angry and brooding, reverting to his basic habits. Perhaps it was a little optomistic of me, but I then had him take a look at the reasoning behind the 'act'. PC had no soul. PC had accidentally tried to kill him while she was the Slayer.
In my mind, he realized that the PC had some serious 'issues' and it was part and parcel of being a soulless Bhaalspawn. Not that he was able to forgive it all that easily, but he did forgive her.
He did it in his basic manner, of course. He allows the PC to dictate or guide his ethics when it came to dealing with his sister, so with something like the PC cheating on him, he turned to another for guidance, which in the story, happened to be Keldorn.
So basically, I think his reaction would depend on the advice he would recieve from whoever is available.