I always understood that there were similarities between Islam and Christianity, so I questioned this book being 'anti-bible' I Googled it and found quotes such as
"The Bible, the Qur'an, and Science is an objective study of the Old Testament, the Gospels and the Qur'an. This book seeks to spiritually unite by highlighting similarities in the texts. It sheds new light and dispels many preconceived ideas in separating what belongs to Revelation from what is the product of error or human interpretation"
and "This book strips away many long-standing--and in the author's view, incorrect--nations about the Old Testament, the Gospels, and the Qur'an, by looking deeply at all three and examining their complex histories and their relationship with science. Emphasizing particularly the Revelation sections of these holy books, Maurice Bucaille inspires the reader to meditate on those factors that should spiritually unite, rather than divide, Jews, Christians, and Muslims."
I believe that his mother has given you a gift intended to show the similarities between your beliefs. It's a show of acceptance and goodwill from her. But instead of looking at it with the positive of finding similarity, you've chosen to take it as an insult to your beliefs.
I can't fault your boyfriend's mother.
Never regret anything that has happened in your life. It cannot be changed, forgotten or undone. So, take it as a lesson learned and move on.