At the very least he's been bloody insensitive. I don't understand why, if it's a group of people, that he needs to take a "plus one" at all anyway .... regardless of whether or not "everyone else" is taking their partners. It's not an activity where a partner is a must ..... such as, hmmm, I don't know ..... an amateur rally driving day where you'd need a driver AND a navigator.
Yes .... they've been racing before. They have a shared interest in something which has gone off the boil for you (understandable, IMO, life moves on, priorities change) but when they've been out before, to indulge this interest, was it on their own or with a big crowd ? I can understand them going out on their own previously as it's not always much fun travelling on your own, or standing about at the event with no-one to talk to but that won't be the case on this occasion will it 'cos there'll be lots of others there.
It feels to me that he can't bear being "billy no mates" in front of his colleagues - which is pathetic, and quite possibly wants to show off to them by showing an attractive, striking woman wants to be in his company. Can't help thinking there's some alpha male posturing going on here - assuming nothing worse is (hope not).
The fact he asks her first, AND is rather vague about who exactly's going, before asking his wife if she wants to come, does ring alarm bells. Asking her doesn't actually mean a lot does it when he knows damn well that due to their daughter, it's almost impossible for her to be out for a full day when his MIL can't provide babysitting for that long. I think asking her is a bit of a red herring TBH, and of course, it's very hurtful.