She wants to marry a man but her mother rejected him

Firstly: 

It is well known that the marriage contract should be done by the woman’s guardian, namely her father, then her son if she has one, then her grandfather, then her brother, according to the order of closeness of male relatives on the father’s side. The mother cannot act as the guardian in the case of marriage, although the mother is greatly entitled to honour and kind treatment, and it is obligatory to obey her, whereas marriage to a particular man is not obligatory. Hence you should try to convince your mother to agree to you marrying this suitor – if he is acceptable in terms of religious commitment and character – and you can point out to her his good qualities and the advantages of marrying him. If she agrees to that, all well and good. But if she insists on rejecting him, then you are obliged to obey her and honour her. 

Secondly: 

The suitor or fiancé is a stranger (non-mahram) to the woman to whom he has proposed marriage, so it is not permissible for there to be any relationship between them or any meeting, shaking hands or being alone together. If your saying that you love him very much is because you have heard good things about him or that has come about without committing any haraam actions, then there is no sin on you, but if it is love that stems from any haraam actions such as looking or spending time together, then what you have to do is repent to Allah, may He be exalted, and avoid the things that lead to fitnah and haraam actions. 

Whatever the case, what you must do now is cut off contact with him and treat him as a stranger with whom you have no relationship, until the marriage contract is done. 

We advise you to fear Allah, may He be exalted, remember that He is always watching, and ask Him to bless you with a righteous husband. 

And Allah knows best.