Gambling shouldn't be the basis for their break ups, there must be some other factors that the couple may have not own up to the public, and secondly, where you aware of the guy Gambling life or he still keep that as a secret to you.
I believe both of you have gamble together and he may be doing other things that you may not be aware of since gambling alone cant be a basis for breakups in a long time marriage as you said.
You're right, they must have been on rough edges for some time now and the lady have been nursing the idea that the guy is not an honest person and have been looking for an opportunity to end the relationship.
Discovering his gambling involvement only ended up being the straw that broke the Carmel's back. I really blame the guy to some extent for not coming out plain in the first place and owing up to his personality. On the other hand it is still possible that the lady's ideology about gambling was very negative which made the guy keep it a secret to her for fear of loosing her.
It is important couples are open to each other to foster an atmosphere of trust and eliminate possibilities of suspicion that can promote resentment within them.