Two of my teachers in school (I'll call them Miss A and Mrs B) were best friends for years. Mrs B became ill and passed away. About a year later Miss A married Mrs B's husband.
This was over twenty years ago, and as far as I know they're still together.
An even worse one was a friend of a friend who I met a few times. She and her sister were very close. Only about a year apart in age. Her sister passed away after a short illness and six months later she left her husband for her now widowed brother in law.
You can only imagine the confusion and upset that caused. Both families had kids and now Auntie was Daddy's girlfriend six months after Mummy had died.
Her ex husband (really lovely guy, a real gentleman) was devastated.
Of course when they both woke up and realised what they'd done, it all ended. She wanted to get back with her husband then, but he said no. The kids were all quite young at the time (pre teens) Her two oldest kids (late teens now) don't talk to her anymore and don't want anything to do with her.