Supporting My Girlfriend's Decision to Reconnect with Her Dad

My girlfriend wants to see her dad for the first time in 10 years. I’m all for it I’m just a little nervous about it. I think it could be good for her because her mom is toxic and manipulative and very emotionally unavailable. She completely shuns emotion of any kind which has really affected my girlfriend and I think her dad will be more emotionally available based on what I know. That being said he is also toxic, he can have a temper and he’s a compulsive liar with a hero complex but she’s not gonna be living with him, only seeing him every now and then so I don’t think it’ll necessarily affect her. Her middle brother (she’s the oldest) sees him relatively often and he is a lot more emotionally okay, my girlfriend tends to get defensive and then shut down and internalize guilt which she is aware of and working on, her brother instead tends to lash out and fight back when her mom is being manipulative and keeps to himself as much as possible. It honestly seems to work out better for him emotionally despite how much conflict it may cause. Her mom has really toxic mindsets so to yield to and accept those while knowing they’re wrong is a lot more toxic than fighting them in my opinion and form what I’ve seen in her family and even just her. That’s maybe a lot of unnecessary information but I like to know the whole situation so I can understand so I figured I’d give everything. My girlfriend is worried about her dads lying rubbing off on her because she used to lie compulsively when she was younger and she has to try really hard not to. She doesn’t lie about important stuff, just stuff that doesn’t really matter and she usually immediately corrects herself. I think the fact that she’s worried about it affecting her is enough for her to be okay but I’m not sure. She’s also worried about her mom taking seeing her dad personally, which she’s already been pushing back with her mom recently and setting emotional and other boundaries so I think she can navigate that and I think it shouldn’t matter. I’m really not sure how to support her other than talk to her and help her decompress when and if she goes and sees him. I’m just wondering if anyone has suggestions on things I can do or say or even signs to look out for so I can better support her and like how I should help her like set boundaries with him because she really struggles to set boundaries and like fall into peoples traps and forget they’re not great people. Also how can I help her get what she needs from him? I think I kinda know how to but any suggestions would be much appreciated because I honestly don’t know how it would feel to be her in this situation and that’s really hard for me because I’m normally really good at reading situations and people and knowing what to expect but I’ve also never met her dad.