I had kind of a similar situation when I was a teenager. The husband sent me letters and gift sometimes after I went to college, and I made sure to write thank you notes addressed to BOTH of them, listing HER name FIRST, and he got the message real quickly and stopped doing it.

Maybe your friend could sometime, in the presence of the wife, say something to the husband like "You know my friends and I all say 'love you' before we hang up, in a friendship sort of way. I guess it's OK if you say it, too, if you want to." If she approaches things like that, always with the wife present, he'll soon get the message. And maybe she could say, in a joking-like tone, something like "What did you mean you didn't want me to tell your wife (or say her name here) that you were chatting online with us?"---again in the presence of the wife.

If she brings everything out into the open like that in her best friend's presence, I can guarantee he'll get the message and it will stop. If not, then he's definitely making a very poor attempt to fit in!