The thing I do not see addressed in your responses so far, is any level of concern about appearances. Yes, I know, who cares what people think?! The old indignant "it's not their business what I do anyway" sort of attitude. But the mature and professional attitude is more in line with "do not give even the appearance of impropriety."
So, I'd say no. Not without the girlfriend or the spouse also in attendance. People WILL talk. And I'm assuming these are professional people you are talking about here. It's unprofessional to do something you know will generate gossip.
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I'm sorry. It does matter what people think.
What is so difficult about getting a 3rd party to attend? Either in lieu of the single guy, or in addition to? It's really not that hard. And it really is that simple. You avoid situations where gossip is sure to ensue, if you value your marriage and your professional reputation (and his relationship and his professional reputation--friends think of their friends' reps as well, you know). If you don't value those things, then why bother asking other's opinions? You seem to want tacit approval. It isn't very professional in appearance, if you don't include others or have one of the "significant other"s present.
You could always just issue an impromptu announcement "if there is anyone who would be interested in going"...
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After your clarification regarding who all was asked and how exactly it happened, I would say that it wasn't wrong to ask the question (assuming that the invitation made it open to the girlfriend to come along as well). But neither is it wrong for the girlfriend to not want him to go without her. It doesn't reveal any distrust in their relationship, or problems with them, either. Just maybe someone who is aware of "protecting" their relationship and not putting themselves in scenarios where things could go wrong. And honestly, a bar scene, drinking and a concert, with a friendly opposite sex couple, and without their other halves, is a situation where it is possible (not likely, but possible) for too much to drink to cause something that otherwise wouldn't happen. AND the issue of appearances (again.. and this could be how things appear if they ran into someone that SHE knows, too).
Not wrong to ask the question. Just be careful not to be judgmental about their relationship if she nixed the idea and he is going along with it. That could be a sign of a HEALTHY relationship as easily as it could be a sign of a bad one.