Sounds like he had issues.
You don't say how long he was like this, months ? years ?
It may be a useful shock, not just for you but him.
Probably not, but it's worth a go.
As for fighting to get him back. To me it begs the question of who you want back ?
I'm guessing that he was once better than this ?
It's practically impossible to fix someone else against their will, but you can provide an incentive to do so.
If you make it seem that this is just an extra big row, then he may not bother to sort out his life, indeed I'd bet money that he's blaming you.
Also the behaviour you describe isn't going to be all that attractive to the "other woman". The occasional trip to the pub and random bonk requires a lower standard of behaviour than a relationship. Although I am of course short of facts, I don't see that lasting very long.
Also, I don't get the impression of someone who will do very well on their own. Easy to see a spiral where he sits on his own with a bottle every night, work suffers and possibly disappears.
Hard to predict that even if you know someone, and of course I don't.
My bet is that the quality of his life is going to degrade, possibly quite quickly. He will of course blame you, her, his job, and possibly even David Cameron, but maybe he will get the message that this is largely self inflicted.
At that point you have some decisions to make...