Certainly the intention counts and there should be witnesses etc however here's what I find fishy. If it indeed was a joke why would his wife go report it to the religious authorities? I mean the chat was between husband and wife,right? Heck Brit jokes with Mrs Expat about murdering her and she,knowing his kidding around, goes to the cops..does that make much sense? Obviously the man wasn't joking, then learned it the hard way and is now trying to play a sympathy card..idiot!
On a side note it is reported in a hadith that the Prophet said among the three things a person should not mock about or mention loosely(unless truly intended) is divorce! And I can see the wisdom in his advice. How many husbands "jokingly" threaten their wives with divorce?my cousin's ex husband would constantly bring up divorce even in the most minor arguments until one day she simply couldn't handle it anymore and put him (and herself) out of his misery by divorcing him!And trust me he deserved it.