As a single male of some repute, I'll say this: Barring a club/house party environment, it's always better for the woman to approach a single man. If she makes the effort to visit that restaurant on a weekly basis (for "A cup of coffee and some quiet time"), she'll be able to find out all of the information that she needs to know.
Now, some extra tidbits of information, seeing as how you've mentioned that you aren't small-town swingers... All advice given past this point is with the assumption that you live in a major city (Major=a city with an airport which accommodates more than a dozen unrelated airlines)
1. If he's more attractive/charming/knowledgeable than normal, it's very likely that you won't be the first couple to have approached him for some "Not ready for primetime action".
1b. Which means that your wife, if she's relatively discreet and he's noticeably interested, can afford to be a bit plainer with her intentions. Not waving a dildo in the air and saying "Are you bigger than this, [insert server name here]?", but a quick comment about meeting the husband and herself at a local bar for a nightcap isn't going to go unnoticed.
2. Take an "eyes wide open" look at your respective levels of attractiveness. This is very important, as (it goes without saying) he's far more likely to agree to a MFM/MMF with an attractive woman. I'm not saying that he's automatically going to reject her if she isn't Cindy Crawford-level beautiful, but presentation does wonders.
{The first few times that I was hit on as a single male, my "interested parties" were of the (warning: racism/sexism) "He's black, so I don't need to worry about showering/having my hair done/wearing clothes that fit, because all black guys want white pussy." variety.}
3. Take note of any sort of age gap. If he's more than 10 years younger than you, be aware that you'll probably have to do a lot of coaching in the bedroom. A lot of coaching. If he's at the "distinguished gentleman" age, it may be the opposite. Either way, whether he proves to be experienced in the ways of the threesome, don't except a sexual dynamo.
3a. For the college student/"new to the city" type, a home-cooked meal can do wonders. My re-introduction to the lifestyle involved the promise of an authentic "native" meal and the first samples of their home-brewed beer. As it turned out, they didn't even need to get me drunk (which was their first plan). As I drank my second beer, the husband went downstairs "to bring up some of the WinterCorn variety" and the wife sat down beside me and started rubbing my hand and asking me if enjoyed the dinner. By the time he came upstairs, she was on my lap and I was enjoying the first real date/makeout session that I'd had in 7 years.
So, long story short, be relatively discreet, be friendly enough to avoid ambiguity and (if he's taken the bait) meet him someplace thoroughly non work-related before laying the cards out on the table.