Frederich is Grace's father. He's an interesting character to study, mostly because he and Grace have very different perspectives on what he was/is like as a dad.
For starters, Grace remembers that when she was growing up, her father would only talk to her when she was "her most sedate, cerebral self" (8.126). In other words, he didn't like it when she got emotional. Readers see that quality of Frederich's play out when Grace finally loses her cool with him and his second wife, Eva, telling them that she has no idea where Jonathan is when they won't get off her case about why he's not at dinner.
Rather than showing concern for this alarming statement, Eva storms off, and Frederich says, "I know you are very consumed with the feelings of your patients, but it does strike me as strange that you never concern yourself with Eva's feelings" (11.197). He then lectures her about checking in with Jonathan. Come on, Dad, read the room.
On the flip side, when Frederich comes to the lake house for Christmas, he brings all of their favorite comfort foods, spends quality time with Henry, and gives Grace a fatherly pep talk about how she'll get through this. Maybe he's just getting softer with age—or the utter catastrophe of his daughter's circumstances makes him realize she needs emotional support.
However, he also tells her that he always wanted more children. He says he loved being her father, watching her learn new things and study the world around her. This recollection of Grace's childhood hardly lines up with Grace's memories of her father being emotionally distant.
So what's going on here? It's possible that Korelitz could have spent less time developing Frederich's persona and ensuring that he remained a consistent, believable character.
Of course, it's also possible that Grace is just thoroughly awful at understanding anybody who's not in her office. She originally believed her parents had a wonderful marriage, as evidenced by the big, flashy jewelry Frederich frequently bought for her mother, Marjorie. "The jewels were a representation of someone doing his best to say I love you and someone else doing her best to say I know" (4.71).
Hmm, not so much. Frederich reveals that he actually bought Marjorie jewelry to apologize after cheating on her. He says they were very unhappy, and when he couldn't be happy with her, he'd find somebody else. He even asked for a divorce twice before Marjorie had a fatal stroke. He feels terrible that Grace based her ideas of a perfect marriage on his relationship with her mother.
It's hard to say whether Frederich is just an underdeveloped character or if Grace truly is that obtuse, but we'd wager it's a little of both.