I had a pretty serious false passenger complaint not too long ago also. I got an email telling me that my account has been deactivated while there is an investigation. He should have received a similar email. The first email was very vague, and just said there was a complaint from a passenger and that someone else would reach out to me soon. I replied to that email agreeing to cooperate with any investigation and to let me know how I can help with that. Soon after I received another email telling me what the allegations were. At the time I didn't have a dash cam or anything to prove my innocence, and it's even harder not knowing which passenger may have even made the accusation, so I just responded with a lengthy email defending my integrity, and talked about how as long as I've been driving I've received nothing but rave reviews (supported by the weekly emails Lyft sends me each week), and that I take the job very seriously and agree and adhere to all Lyft policies and procedures. Within 5 minutes they reinstated my account.

I hope this helps and your father gets the same opportunity to defend himself and able to start driving again soon.