My friend has done something bad in her past. She regrets it, and when she told me about it she made me swear by Allaah that I would not tell anyone, but people at the masjid are suspicious. They keep asking me about her and whether she has done that something. If i say, "I do not know," then that would be a lie, because I do know. If i say, "No," then that would be a lie as well, and if I say, "Yes," (the truth) then that would mean that I would have broken the promise that I made my friend (in Allaah's name). What should I do?

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. 

This sister has done well by regretting the sin that she committed; she should make this regret a sincere repentance, fulfilling all its conditions which we have mentioned in fatwa 86527. Along with repentance, she should conceal her sin, so it is not permissible for her to inform you or anyone else about her sin.

Abu Hurayrah narrated that the Prophet said, “All (those of) my nation are forgiven for their sins except those who sin openly or disclose their sins to the people. An example of such disclosure is that a person commits a sin by night, and though Allaah screened it from the people, he comes in the morning, and says, 'O so-and-so, I did such-and-such (evil) deed last night,' though he spent his night screened by his Lord (none knowing about his sin) and in the morning he removes Allaah's screen from himself.” [Al-Bukhari and Muslim]

The people of the mosque or others have no right to investigate whether or not she committed that sin. The Prophet said, “Among the good traits of a Muslim is leaving that which does not concern him.” [At-Tirmithi]

So it is a must to advise them and remind them that they are not permitted to investigate this matter and search for the faults of others.

Abu Barzah Al-Aslami narrated that the Prophet said, “O those who are Muslims by tongue whereas faith has not entered their hearts! Do not backbite Muslims or pursue their faults, for whoever pursues the faults of his Muslim brother, Allaah will pursue his faults, and whoever of whom Allaah pursues his faults, He will disgrace him even if he hides himself in his own home.” [Abu Daawood]

Therefore, it would not have been permissible for you to inform them about her past sin even if that sister had not made you promise not to tell anyone, so what should the case be when she actually made you make such promise?

Also, it is not permissible for you to lie to them because lying is not permissible except in case of necessity. However, you may use tawriyah (saying something which has more than one meaning and intending a meaning different from what the listener is likely to understand) by telling them, for example, that your friend did not do it, meaning after she repented from it.

For more benefit, please refer to fataawa 84728 and 88412.

Allaah knows best.