What is the ruling on someone who causes trouble between an engaged couple so that he will call it off and she can snag him?

Praise be to Allah.

Firstly: 

Causing trouble between Muslims is a major sin that corrupts intentions in people’s hearts, spoils relationships between people, and spreads mischief in the land. 

It says in al-Mawsoo‘ah al-Fiqhiyyah (5/291): 

It is prohibited to cause trouble and spoil relationships among Muslims, for two reasons:

1.in order to preserve unity among the Muslims

2.out of respect for their dignity, because Allah, may He be exalted, says (interpretation of the meaning): “And hold fast, all of you together, to the Rope of Allah (i.e. this Qur’an), and be not divided among yourselves” [Aal ‘Imraan 3:103]. 

Hence reconciling between people is one of the best of good deeds, and causing division among people is one of the gravest of major sins, as the Prophet (blessings and peace of Allah be upon him) said: “Shall I not tell you of something that is better than fasting, prayer and giving charity? ” They said: Yes indeed. He said: “Reconciling between people, for causing division among people is the ‘shaver’ (that shaves one of religious commitment).”

Narrated by at-Tirmidhi (2509); classed as saheeh by al-Albaani in Saheeh al-Jaami‘ (2595). 

Hence the Messenger (blessings and peace of Allah be upon him) forbade seeking out the faults of the Muslims, and he forbade backbiting, malicious gossip, suspicion, mutual hatred, mutual envy and everything that may lead to causing trouble among Muslims. End quote. 

This prohibition is emphasised if causing trouble between people is done to spoil that which could have been the means of bringing them together, instilling love between them and uniting them, such as marriage. 

Secondly: 

Causing trouble between people usually involves telling lies, backbiting, malicious gossip and slander, all of which are major sins. Please see the answers to questions no. 23328, 101776 and 99554 for information on the negative impact of these blameworthy characteristics and how to repent from them. 

Thirdly: 

It is haraam for a Muslim woman to cause trouble between a man and his fiancée so that she may snag him for herself, for this comes under the heading of bad conduct and blameworthy characteristics. Al-Bukhaari (6601) and Muslim (1408) narrated that Abu Hurayrah (may Allah be pleased with him) said: The Prophet (blessings and peace of Allah be upon him) said: “A man should not propose marriage to a woman to whom his brother has already proposed and he should not outbid his brother. A man should not marry a woman if he is already married to her paternal aunt or maternal aunt. A woman should not ask for her sister to be divorced so as to deprive her of what is rightfully hers and so that she may be married in her stead; rather she will have what Allah has decreed for her.” 

An-Nawawi (may Allah have mercy on him) said: 

What this hadith means is that it is prohibited for a non-mahram woman to ask a man to divorce his wife and marry her instead, so that she gains the maintenance, kind treatment, intimacy and so on that belonged to the divorced woman.

End quote from Sharh Saheeh Muslim (9/193) 

Al-Haafiz (may Allah have mercy on him) said: 

This is quoted as evidence for the prohibition on a woman proposing to a man who has already proposed to another woman. This hadith makes the ruling concerning men the same as the ruling concerning women with regard to proposing marriage. The scenario is: a woman proposes to a man, and he responds to that proposal, then another woman comes and encourages him to marry her, and tries to make him lose interest in the one who came before her, whom he has already agreed to marry.

End quote from Fath al-Baari (9/200) 

Fourthly: 

The issue does not have to do with whether all the parties involved in this issue know what is happening or not, because worldly matters may be hidden. But on the Day when all secrets will be exposed before the Lord of the Worlds, and what was hidden in people’s hearts becomes manifest, what answer will those who caused trouble and spread malicious gossip have before Allah? 

What the woman who has done this must do is hasten to repent, and part of complete repentance is putting right that which she has spoiled and admitting to each party what she said about the other, so that they will understand the situation, then if they want to they can get back together and go ahead with their marriage, or if they want to they can remain as they are now. 

If it is too difficult to state what really happened, then that can be done by way of hinting or via a third party, striving to set things straight and explain what really happened. 

Please see also the answers to questions no. 14092 and 178714 

And Allah knows best.