Are you flying as well? International? Domestic?
If not, then you would be limited to any land-side locations. There are a fair amount of restaurants and coffee shops on level 3, land-side, and a food court on level 1, door #10. These areas can be noisy and congested at certain times of the day, but you can find quieter public areas if you look around a bit, especially on level 1 and level B (one below level 1). And, in the main passenger terminal, the hall where people check in, at the back are several levels with an observation deck, and a few other spots.
Was your wife planning on checking bags through to CNX, clearing mid-field Immigration at BKK, and then Customs at CNX?
Obviously, for you to meet her land-side she would have to clear Immigration and Customs in the main-hall, on level 2.