A conference of two people? I mean in our company it's absolutely normal (and even required by HR) to have at least one, better two meetings per year which are only between the boss and you. These meetings are at first for certain work things and on the other hand meant as a review of the past year for both sides, like what did the employee do good / bad, what can the boss improve. As these talks are obviously extremely confidential there's never another person present (except if you insist on someone from the works council to be present). But as soon as you are invited you are informed about the "why" and it's sort of a mass thing as every department boss has to do these talks in a rather short time span.

If you aren't sure what's going to happen, just make your wife ask her boss what the meeting is going to be about so she can prepare herself.