My inlaws did similar - no divorce but split up as soon as the kids left - it was mostly my MIL that wanted him out for no good reason.
Then my FIL was living like a 20 year old with his own apartment. Big screen TV, mattress on the floor, etc. It was pathetic.
Very WTF.
My MIL bought a condo in a retirement community. She had no car (always relied on my FIL to drive her everywhere). So she was riding the shuttle and taking public transport. Plenty of friends and family but would not stop bitching she was lonely.
So she took him back in. He sleeps on the futon in the living room and she in the bedroom.
Still bitches about him and in total denial that for better or worse they are joined at the hip.