Run away

Assuming there is nothing more to this story, i.e. she didn't steal, and her employer didn't abuse her, then her visa is automatically revoked when her employer reports her to immigration.

If she leaves HK soon without doing anything illegal, then nothing will happen to her. She broke a contract not a law. If she tries to stay in HK on a voided visa, she would violate the conditions of her visa, and she will be deported when she leaves or is caught. Whether there will be jail time or not will depend on what she does and how long she stays in HK without a valid visa. If she is already out of HK, she will be refused entry on the same visa. It will be hard for her to get another visa in the future.

If she run away because her employer did something wrong, she should find some legal help to enforce her own rights, and file the case quickly.

Sounds like your friend had a bad employer, but not bad enough to sue in court. She needs to leave Hong Kong soon, or it may become an issue. It would also be better if she sends a letter to her employer and agents saying she resigns due to unacceptable working conditions.