Assalamu alaikum wa rahmatulahi wa barakatuh sister,
It is a shame that 2 years on she is still being punished by her family for a mistake that she made some time ago. She is clearly feeling bad enough already and is doing the best she possibly can give the circumstances by repenting for what she did and not having anything to do with man. Alhamdulilah, despite them being so cold with her, she has been able to get support from you in the matter.
Regarding her situation, she is doing all the right things which in sha Allah will be viewed favorably by Allah as He loves to forgive those who repent.
Of course, she will feel guilty for committing a sin, as any Allah fearing Muslim should, but the situation with her family is only prolonging things and making it worse for her which will make it difficult for her to move on.
There are different ways for the family to deal with such a matter depending on the person themselves and the steps they have made after. Being cold towards them will make them feel bad for what they have done and can contribute towards putting them off committing the sin again in the future, which can be a good thing to a certain extent.
However, when this attitude is prolonged it can have a detrimental effect on the person leading to depression as has happened in this case. This is not helpful for the person as it can send them in the wrong direction and to feel unworthy and undeserving of anything, or in the other extreme, to rebel and leave Islam and go back to the sin. Effectively, this technique may be the useful short term, but if continued can have the opposite effect.
On the other hand, a more gentle approach can be taken where the family talks to the person gently. Talking to them from a place of love for them for the sake of Allah. They can educate them and let them know their concerns about their behavior. This way, the family get a chance to state their disappointment, whilst supporting them in making positive changes.
This way, the person is more likely to make positive changes and not commit the same sin again. Let them know that they are talking to them out of love. That they love them enough not to want them to have to face the wrath of Allah. In this particular case, this approach may be especially useful since the girl in question does feel so lonely which you feel lead her to zina in the first place. If she feels supported, she will feel less lonely and therefore less likely to seek companionship elsewhere in a way that may lead to sin.
May Allah reward you concern and guide everyone on the straight path.