All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
We ask Allaah to grant you good reward for what you undertake, that is, arranging matches between young Muslim men and women. But you should not try to help a girl to get married without the consent of her guardian, as marriage without a Waliyy (guardian) is invalid. If you do so, you would be participating in committing a sin. Allaah Says (what means): {Help you one another in virtue and piety, and do not help one another in sin and transgression. And fear Allaah. Indeed, Allaah is Severe in Punishment.} [Quran: 5:2]
The majority of scholars believe that marriage without a guardian is invalid because of the evidence from the Quran and the Sunnah. Allaah Says (what means): {And do not marry idolatresses until they believe (i.e., worship Allaah alone). And indeed a slave woman who believes is better than an idolatress, etc, even though she may please you. And give not your daughters in marriage to the polytheists until they believe in Allaah.} [Quran 2:221] Allaah, then, has placed the responsibility of marrying off the girls into the hands of the Waliyy.
Allaah also Says (what means): {And when you have divorced women and they have fulfilled the term of their prescribed period, do not prevent them from marrying their (former) husbands, if they mutually agree on reasonable basis.} [Quran: 2:232] This is yet another evidence that the woman cannot marry herself off. There is also sound evidence from the Sunnah that the marriage that lacks the consent of the Waliyy is invalid. It is narrated from Ahmad, Abu-Daawood, At-Tirmithi and Ibn Maajah from the Hadeeth of Ibn Abbaas and his father, as well as Abu Moosaa that the Prophet Muhammad, sallallaahu ‘alayhi wa sallam said: "Marriage is invalid without the Waliyy." Also, it is related via Aa’ishah that the Prophet, sallallaahu ‘alayhi wa sallam, said: "Any woman who gets married without the consent of her Waliyy, her marriage is invalid, her marriage is invalid, her marriage is invalid." [Abu-Daawood, Al-Tirmithi and Ibn Maajah] Also, Al-Daaraqutni narrated from Abu Hurayrah that the Prophet, sallallaahu ‘alayhi wa sallam, said: "A woman has no right to marry another woman off, nor has she the right to marry herself off as it is only an adulterer/fornicator woman who marries herself off."
It is a fact that the woman is very emotional by nature and this is why the Sharee'ah gives her guardian the right to marrying her off.
You should help in guiding people to submission to Allaah The Almighty, and beware of opening a gate of sins for yourself when your intention was to preach the good. As a result, you should try to convince this girl to obey her parents first and not to do anything that would go against their consent and will.
Allaah Knows best.