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, you should know that it is wrong for people to believe that happiness is only achieved through the worldly pleasures which a person possesses, like money, beauty and the like. Indeed, we see many people who are blessed with the above bounties and it was a punishment for them and a reason for their unhappiness. Allaah Says (what means): {So let not their wealth or their children impress you. Allaah only intends to punish them through them in worldly life and that their souls should depart [at death] while they are disbelievers.}[Quran 9:55]
The real happiness is to have faith and perform acts of obedience, Allaah Says (what means): {Whoever does righteousness, whether male or female, while he is a believer— We will surely cause him to live a good life, and We will surely give them their reward [in the Hereafter] according to the best of what they used to do.}[Quran 16:97]
For more benefit, please refer to Fatwa 83577.
It is forbidden for a Muslim woman to have an affectionate relationship with a marriageable man, as this is the practice of the people of the pre-Islamic era of ignorance. Also, it not befitting for a believing woman, who seeks to please Allaah and enter Paradise, to think to tread the path of dissolute women, let alone doing what they do. For more benefit, please refer to Fataawa 90336 and 85254.
It should be noted that everything in this universe runs according to the Predestination of Allaah; it is He Who provides for people and it is He Who holds back provisions for some other people. He does so out of His absolute Knowledge and Wisdom. It is not life that gives or does not give.
Allaah gives worldly pleasures and bounties to those whom He loves and those whom He does not love, but He only gives faith to whom He loves.
Anyway, all the matter [whether a person is given from the bounties of Allaah or depressed from it] is a test from Allaah to His slaves, and then the recompense [reward in Paradise or punishment in Hellfire] will be on the Day of Judgment.
Therefore, we advise your sister to be patient and to earnestly supplicate Allaah to predestine what is good for her and bless her with a good husband.
She may seek the help of some righteous sisters in finding a pious husband for her.
Finally, it should be mentioned that it is an abominable act and a grave sin for some girls to have lovers, and what they get from them is evil in this world and a torment in the Hereafter.
Allaah Knows best.