Missing the Title of the book- mystery fiction

Book starts with a couple that meet at a cafe or diner. The main character is in her 20s when a man asks to take her out. He’s in his 50s. The story fast forwards to 20 years later. They are married and attend an event. Later that evening, she walks in and he’s in bed with another and has a heart attack. The main character is left out of his will and she decides to investigate the matter.

Please help me find the title of this book. It’s been driving me crazy. I read it as a teenager and never finished it. Now I’m in my late 20s and I’d like to finish it.

Sounds like a mary higgins clark book that I read called "I heard that song before". It isn't that one, but could it be by her? She has a lot of books

It’s not any of her books unfortunately. I did just check.