Would you be miffed if it was another male he was having dinner with? I'm just curious because I know people who do not like their spouse doing anything with anyone that doesn't include them(something else I find odd when in relationship that you can't have down time away from your spouse) or if it is again just because it is with someone of the opposite sex.
Generally speaking, I would find it weird for 2 guys to make plans to go to dinner together unless it was part of some other activity:
Hey Bob, I've got a spare ticket to the ballgame, wanna go? We'll grab supper on the way.
Hey Bob, I wanna see the new Stallone movie & my wife doesn't. Wanna go? We'll grab supper on the way.
Hey Bob, wanna go fishing on Saturday? We'll find somewhere to eat along the way.
Hey Bob, highway's shut down for a wreck. Wanna grab a pizza up the street & wait it out with me?
Any of the above sort of "make sense" to me, ditto lunch at work or any meals related to business or business travel. Just making plans to go to dinner & only dinner with another dude? Weird. As a guy, a planned one-on-one dinner is a date. Period. Probably why I wouldn't do it with a female other than my wife, either
Now, I HAVE done quite a few one-on-one lunches while at work with female coworkers. But, there were always other people invited. If it turns out the only one who can (or wants to) go is a female, I'm still going. But, I wouldn't set out to make it so.