Upvote:1
The key is to be truthful. In countries with legal divorce, lots of people are separated while the divorce goes through, and may be in other relationships at the same time, so I don't think that this situation would raise any particular red flags with the Schengen authorities. (Of course, since he is coming to visit a romantic partner, this may generate the usual concerns about overstaying.) Based on what you have said, I think the most honest answer would be to check that he is married, but then specify that he is separated but in the process of getting an annulment.