She sounds like my wife during our early years. Unfortunately that's not something that works well with this lifestyle. One of the hardest hurdles my wife had to overcome was her inability to talk about her feelings and emotions. It wasn't something her family did so she wasn't used to it.
With a lot of talking, she slowly realized that how she was trying to process through her feelings and emotions wasn't working. When there are three or more people involved it's virtually impossible to deal with things on your own.
I would strongly encourage your SO to talk to his wife about this. Maybe a sit down with the 3 of you isn't in the cards just yet, but she's got to feel more comfortable telling her husband how she's feeling and what's going on in her head.
Trying to guess what's going on will only lead to someone getting hurt or upset.
Live life to the fullest 'cause you never know if you're gonna wake up tomorrow!