"Happy" almost becomes a ridiculous word in this context. It's like "love". It means, what? IRL, rarely are those concepts reducible to happy/not happy, love/not love.
So maybe the day that picture was taken she was feeling bereft, empty, malnourished in her marriage ... but also she was maybe feeling joy and comfort, trust and friendship in the familiarity of her family.
I can remember days in the middle of our struggles when I had no idea how it was going to turn out, if I was ever going to get what I needed from the relationship; those days could *still* be and often were full of joy, laughing my head off at some mutual silliness with my h.
So when you (humanlike) are looking in the rearview mirror at this chaotic mix of seemingly-contradictory emotions, what determines which ones stand out, seem "real"? Nothing more than whatever emotional filter you happen to be using at the time, be it "We are best friends and co-parents; nothing more", "We have a great marriage and I adore you", or "Everything sucks and I never loved you". Which probably explains a lot of the (really infuriating) WAS rewriting and general flipfloppiness. IMHO.
"Show me a completely smooth operation and I'll show you someone who's covering mistakes. Real boats rock." -- Frank Herbert