I don't see a problem. He got married. It's well known that if you get unmarried, you split any assets acquired during the marriage. It has been that way for a long time, so it's not like it was a surprise to him or anything. If he didn't like it, he shouldn't have agreed to marry her.
It's like if I sign a contract with a business partner to go 50/50, then ten years down the line decided I can't stand them, and want out. I don't get to bitch and whine about the 50%, just because the business is worth $150M now and it was worth jack shit when we started. Even if I feel like I did all the work to get it there. After all, I agreed to it in the first place.
Marriage is a contractual agreement like any other. Just because you happened to be thinking with your penis when you signed it, doesn't mean it's not valid.
Now, there ARE situations where I think it is inequitable (like if your spouse runs off with someone else, they shouldn't be entitled to 50%), but in this case, he was the one dicking around, so he doesn't have any cause for complaint.