I know personally a very similar story. A guy had a wife who he loved, but she had a stroke (IIRC). He took care of his wife, but had no real relationship with her anymore as his wife couldn't speak, etc. He got a girlfriend who he could see several times a week for companionship. When I met him and her, this was the situation already for many years.

Eventually his wife died as the physical ailments took a huge toll on her body. He married his girlfriend and they were very happy.

His girlfriend was MUCH younger than he was. About one generation younger (IIRC) but not two as in your story. We weren't close friends, but AFAIK, age was never an issue in their relationship, either before his wife died or after.