I don't have a lot of experience in this aspect of relationships,
but I'd assume your best bet is to talk with your dad, Ask him what he think of you g/f and try to explain to him how you feel about her not being accepted.

ifyalways:

Hes a kid cos he :
does not know if hes compatible with his lover.
said his dad is 80% right,indirectly meaning hes not compatible with his lover.
If hes yet to know if him and his partner are not compatible,then hes a kid and have no biz attempting marriage yet.

The fact that he said he knows that his dad is right is enough to state that the guy believes the girl is not compatible with him but he would rather go ahead withe the relationship anyway.

Love and compatibility are two different things. Love is not ALL it takes to sustain a marriage

jumie:

If you say your dad is 80% right, then it means that you have already made up your mind to listen to him.

However, I believe if you truly love your girl, you should find a way to convince your dad that you love her, she is what you want and you both can make a good couple.

Overall, you need to weigh all your dad's point first. Pray also and involve your mum, siblings etc if you feel otherwise.

Thats it!

You mean he should go ahead and marry the girl because he loves her even after confessing that they are both incompatible?

Easy to see why there are lots of miserable marriages these days