Can you help my friend out. She is going nuts trying to remember this book.

Okay book loving people! I recently read a book and I can’t for the life of me figure out the title. I only know a few details about what happened in the book, but not entirely sure if I am correct lol It has been bugging me for about a week now! Here it goes...

In the book, I remember a girl going to her grandmothers house by a lake or ocean and she unveils secrets by reading through her journals. She comes across a number that she calls and it’s a grandson to her lover 🤷🏻‍♀️ I think. She then goes and visits someone at the nursing home to ask about the secrets about her grandmother. I know this is VERY VAGUE, but that is all I remember. 🤪🙄