well, ok! i say you have 2 options..

A) Just do what u said, talk to her before this dinner, and as you two will be friends, you two can hang out at the dinner and have a good time.

or

B) this is the fun option but if u have trouble talking to her because of nervousness it may be a bit to much ( which is no big deal, you gotta start somewhere ) but anyway.. since she knows SOMEONE likes her at this dinner you and your friend should get creative and spring it on her that it is infact, YOU who likes her, sure it gets everything out there all in one moment but hey, you DO want to find out how she feels right? this one can prove to be very fun though, as im sure you and your friend can come up with some creative ideas to pop the answer to her that is you all along that was "that guy that likes her" just think of some ways, plus, if she does have feelings for you, she will be flattered by the effort you put in to doing it.