She didn't have to admit this, she can just STFU about having sex. If Muslim countries allow pre-marital sex then their countries would turn into Indias where women get raped in masses.
Sharia law impose strict conditions in applying adultury punishment which is impossible to prove unless:
1- The man and woman who committed adultery have not to be married.
2- There must be at least four witnesses who are sane, rational, impartial, adult, men who saw the adulterers committing adultery with their own eyes and must see it in complete,
3- The adulterers have to admit of committing it.
That shows, that it's impossible to prove adultery unless the adulterers themselves commit it in public and admit it. So, here they are not just committing a great sin but also calling for it, the point of punishment is to limit committing such things, or otherwise people would rape and have sex in streets. But those who commit it, they just get a room and STUP and no one would bother them not even in case three men saw the.