His GF is not a bitch, she's actually really nice. But she is quite forceful sometimes (to the point of disrespecting him in front of us) about her opinions i.e. never letting him get the last word, always having to be technically correct about every single thing. He's a nice guy, so I know she has manipulated him into thinking a certain way, and now he's justified it in his own head that this was all his idea in the first place.
I can't help but notice that there were way more negatives than positives in that paragraph. Especially the part about never letting the other other partner get the last word and always needing to be right. I knew a couple, they were relatives, who were like that. Except switched around, it was the husband who was always right and he had a patient, long-suffering wife. She had tons of health problems later in life and eventually passed away, and to this day I believe it was from the stress of listening to his crap all the time and never being able to speak up and express herself enough.
There was a story a while back, it wasn't about a future wife, but a guy trying to save his buddy who had a female friend that just used him all the time because he liked her.
With both of them present, the guy spontaneously asked her questions like, "When is my friend's birthday?" "Who's his favorite author?" and things like that.
She couldn't answer a single question.
The friend's eyes widened and it hit him how little she really cared about him. Helpful him tremendously in getting over her and cutting contact.
Not sure if that would work in your friend's situation. However, even if your friend's girlfriend knows the answers, another important aspect is how she feels about those things. If she says stuff like, "Oh yeah, he's always reading those stupid books," it's really telling.
A related concept was in this great article Masters of Love:
Throughout the day, partners would make requests for connection, what Gottman calls “bids.” For example, say that the husband is a bird enthusiast and notices a goldfinch fly across the yard. He might say to his wife, “Look at that beautiful bird outside!” He’s not just commenting on the bird here: he’s requesting a response from his wife—a sign of interest or support—hoping they’ll connect, however momentarily, over the bird.
The wife now has a choice. She can respond by either “turning toward” or “turning away” from her husband, as Gottman puts it. Though the bird-bid might seem minor and silly, it can actually reveal a lot about the health of the relationship. The husband thought the bird was important enough to bring it up in conversation and the question is whether his wife recognizes and respects that.
People who turned toward their partners in the study responded by engaging the bidder, showing interest and support in the bid. Those who didn’t—those who turned away—would not respond or respond minimally and continue doing whatever they were doing, like watching TV or reading the paper. Sometimes they would respond with overt hostility, saying something like, “Stop interrupting me, I’m reading.”
These bidding interactions had profound effects on marital well-being. Couples who had divorced after a six-year follow up had “turn-toward bids” 33 percent of the time. Only three in ten of their bids for emotional connection were met with intimacy. The couples who were still together after six years had “turn-toward bids” 87 percent of the time. Nine times out of ten, they were meeting their partner’s emotional needs.
The last resort would be the nuclear options. Get a background check on her. Maybe she's got deal-breakers in her past that she's hiding from him. Money problems, mental health issues, etc.
The last-ditch option would be to not support the wedding at all and not attend. "Look, you're my friend and I care about you and I want you to be happy. Honestly, I don't think she's the girl that will make you happy. As your friend, I care about you too much to support this decision. I won't be attending the wedding. But my door is always open if you want to re-connect." Just be prepared to lose your friend for a couple years until the divorce happens. Or forever.
Tough situation.