Yes that's far too young to make her own decision, they are too young at that age to understand things like the long term effects of not knowing your dad. A child of that age if offered sweets or dad would pick sweets every time, but then grow up with issues related to the fact they didn't see dad. Sorry to hear he struggled so much with it, I think there's no support for dads in this situation and society doesn't place any importance on it at all, it disgusts me.
I'm really glad to hear he is going to go for a court order. At that age I think kids can be told be a vindictive parent 'your dad didn't want you', and poisoned, and to me a court order is something to say to a child of that age, I do want to, I want to very much and in fact so much I asked the judge to give me this bit of paper that means you get to see me.
I think that the daughter might WELL have real reservations about it all, and feel confused or scared or resistance, but again that's not to say it shouldn't happen, to me that absolutely means it should. A child should, wherever possible, have contact with their parents and know that the are loved by them so I think what he's doing should be praised. It's great you are supporting him. My OH had to fight for a court order, and it was the best thing he did, his daughter has a loving very close relationship with us and enjoys her time very much indeed.
I think with a bigger kid like that he could well acknowledge all these things, that it does feel weird, be prepared to answer questions about why things happened (to a basic child friendly level of detail, I wasn't every well for example). I think kids can read honesty and if he is honest about the fact that he really wants to see her and be involved because he cares about her that his daughter will benefit from that over time.
My SD has been told her whole life to hate me but actually she loves me dearly now, because I just kept being honest and kind with her, so I say don't worry about the resistance and push for it. I think that if they court has a sensible story as to why things are as they are and a willing dad wanting to be involved they will see the positive in that.
For us we had to go through the motions of 'mediation' requests at first which of course we knew would be rejected and ignored and messed around, and then go right to the court day to get the order, in all it is fair to say it took most of a year but it depends what point you're at. We had to pay for legal fees and in all we probably paid around £1k which we had to borrow but it's been worth it. It is best for the child to have a plain set of dates and routine to understand in my experience, rather then mum 'granting' days at whim (when they need a babysitter) and imposing rules which stifle the development of a normal father daughter relationship, such as other family members needing to be present, this friend can not see them this one can etc.
Good luck to you