She falls into sin time after time, and she thinks that her fiancé is the cause of that

No File Exists
Check below answers in case you are looking for other related questions:
If he marries his friend’s wife after his friend dies, is that regarded as a betrayal?
It is haraam for a man to be alone with his fiancée or kiss her

No File Exists
Related Answers:
Can a Suitor Lie to the Non-Muslim Family of the Girl He Wants to Marry and Say He is Unmarried
Ruling on speaking to one’s fiancée
Marrying someone who is infected with and is a carrier of hepatitis B
If he marries his friend’s wife after his friend dies, is that regarded as a betrayal?
It is haraam for a man to be alone with his fiancée or kiss her

Recommended answers for you:
Questions about zakaah on jewellery and how it should be paid and who should pay it
Will children who die young go to Paradise or Hell?
What should he do if he gets urine on his clothes?
Why a woman should not take her husband's surname
The reward of one whose child dies and he bears that with patience