Yes, if a woman's husband is in another relationship (i.e., having an extramarital affair or involved in a romantic relationship with someone else), she can file for divorce. The specific laws and procedures for divorce vary from country to country and state to state, but in general, infidelity or being in another relationship can be grounds for divorce.
Infidelity is often considered as a form of marital misconduct, and it can be cited as one of the reasons for seeking a divorce. However, some jurisdictions have "no-fault" divorce laws, which means that a spouse can seek a divorce without having to prove any specific fault or wrongdoing on the part of the other spouse. In a "no-fault" divorce, irreconcilable differences or the breakdown of the marriage may be sufficient grounds for the divorce.
If you or someone you know is considering divorce due to infidelity or any other reason, it is essential to consult with a family law attorney who is familiar with the laws in your jurisdiction. They can provide guidance on the legal process, your rights, and the potential implications of filing for divorce. Additionally, seeking emotional support from friends, family, or a therapist during this challenging time can be beneficial.