When Marcus and Rachel are sitting on the bench in the park, the woman sitting on the bench next to them can be seen reading the book, "Something Blue" by Emily Giffin - "Something Blue" is the sequel to the book "Something Borrowed".
The book has a character named Hillary, another lawyer and works with Rachel on the same law firm. Hillary is the one who guessed Rachel's secret affair with Dex when the latter sent red roses. Hillary's character isn't present in the film's version.
An earlier comment mentions the woman on the park bench next to Rachel and Marcus reading the book "Something Blue" by Emily Giffin, the sequel to the book "Something Borrowed". This woman is Emily giffin herself!