I once heard someone suggest that (other than a family wedding) only the spouse who is friends with the chassan or kallah should be invited and attend, as opposed to automatically inviting the other spouse, too.
I disagree with this. Proper etiquette is that if you are inviting a married person, you invite the spouse as well*. The other spouse doesn’t have to attend, of course, but it is only proper to invite them.
The Wolf
* Barring unusual circumstances, of course — such as a technically still-married person in the midst of getting a divorce.