It's a respect and acceptance issue.
It's probably not a case of "if i have nothing better to do, then i'll do you." or at least i hope not.
It may very well be that his wife goes ahead and makes plans on behalf of both of them without advising him as such. So he checks to ensure that this hasn't happened. He may have many talents, but I highly doubt psychic ability is one of them.
However, in my opinion, if she hasn't told him of plans that she's made without his input, and she is truly is accepting and respectful of his other relationship, then she should understand if he's gone ahead and made other plans instead and hence either opts to attend alone or reschedule.
Communication is important on all sides.
Make plans, advise the other party immediately. It's really not asking for a lot.