Being a reformed control freak I have shared with friends the "facts" about their significant other because I surely would want to know myself! Did this twice with cheating lowlife men with my girlfriends and it did not end up well for me...of course they both reconciled and it became uncomfortable for all...after all I knew the truth!
A girlfriend of mine was in a toxic relationship with a narcissist manipulator and I was her rock getting out of her relationship with him too... yep... they are back together! Just saw them both socially and it was a bit awkward but I greeting the jerk and didn't say a word to gf who has complete rocks in her head.
I have had my share of rocks in my head and there is no fathoming the ties that bind in toxic love relationships.
My advice... steer way clear of direct involvement. If you have a HP pick up the red phone and if your Dad is checking in as well with God the matter will come to light sooner or later. And when he knows about her fb changes it may not make a whit of difference... he knows who she is as he lives with her full time.