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.
First of all, we ask Allaah to guide us all and this sister to His straight Path and enable us to perform good deeds and be steadfast on His Religion until we die.
We hope that by now she knows the danger of having a bad company and she should avoid this in the future. We also hope that she has sincerely repented from the sins she had committed, like neglecting her prayers and talking to marriageable men. For more benefit, please refer to Fataawa 86527 and 83055.
Of course everyone sins, but it is not permissible to consider this as an excuse to repeatedly sin, rather a person should immediately repent.
The Prophet said: "I swear by Him in Whose Hands my soul is (i.e. Allaah), if you do not commit sins, Allaah will replace you with other people who would sin and seek His forgiveness, and He will forgive them." [Muslim]
This girl should know that not marrying the boy whom you mentioned in the question could be a test for her or a punishment for a sin she had committed. It could also be that Allaah Willed good for her by not marrying him. Therefore, she should not be preoccupied by what happened in the past; rather, she should look to the future and ask Allaah to bless her with a righteous husband. She may seek the help of some righteous women; and it might be that her repentance and the good deeds she performs will be a reson for achieving her aspiration.
As regards how to forget that boy, then she may refer to Fatwa 84544 which is about how to treat love sickness, and she may refer to Fatwa 86921, for the means of achieving real happiness.
Concerning her bad female friends, if they become righteous, then she may still keep company with them, otherwise she should avoid them especially if she does not have enough religious knowledge and faith to influence them and protect herself from them.
For more benefit on the danger of keeping a bad company, please refer to Fatwa 88953.
Allaah Knows best.