Its only bigamy if he is legally married to the 2nd wife. Which I don't think he is.
To get divorced in hk both parties are informed. Even if she didn't consent, she would have received divorce papers from the court telling her her husband had applied for a divorce.
Either the 2nd wife is on a tourist visa and I doubt immd will let her do many visa runs so the problem may go away soon.
Or she's got a legit employment visa- which there is nothing you can do about.
Or your dad has managed to get her a work visa illegitimately- which immd could look into.
Or your dad has brought her in claiming she is a another relative somehow- which immd could also look into.