Oh, I get that. I can sympathize (sort of) with him wanting human contact and something simple after the entire thing with Milla, which is why I would be willing to forgive him being tempted. The fact he went through with it is too much. That goes from 'human and fallible' into 'dick' territory, no matter how low he was feeling. I have no idea what Bru expected to accomplish with that. Readers are (often) able to forgive cheaters, but less so if they are cheating on a spouse and they're certainly not going to be quick to forgive someone who cheats on an already victimized and sick spouse.
At the end of the day I have more sympathy for everyone else in the book than Matt*, and that's a sign its gone too far and Bru's lost the essential balance. I think Miller was much better, his Matt could be a dick, but he was still essentially a good man. Bru's Matt...he's got to do a lot of work if he wants to even reach 'decent' man.