I really didn't mind Angel/Nina relationship because she was only in a few episodes but what do you think how Buffy would have reacted on learning Angel had a new girlfriend?
Well, I think Buffy and Angel are both pretty jealous when it comes to each other’s partners. That seems to just be canonically true. Even though Angel left Buffy to go find someone who could take her into the light he wasn’t thrilled about seeing her with Riley. (Even less so with Spike) I think it’s human nature that if you still have feelings for someone, you get jealous when you see/hear about them with other people.
Similarly Buffy was jealous with Angel of both Cordelia and Faith. She seems jealous as late as s7 of BtVS when she hears that Faith mind walked with Angel. I can imagine she would be jealous at knowing that he had a new girlfriend in season 5. I even wrote a fic that addresses some of that: The Work We Do. Spoiler alert she isn’t really excited about him shacking up with another girl.
But ultimately Nina wasn’t going to be the love of Angel’s life. So I think the question that really bugs me the most, the one that sticks at my side and I wish we had an answer for is: How would Buffy feel about Angel and Cordy and whatever feelings he had for her? Because even though that wasn’t a relationship, the feelings between Angel and Cordy ran deeper than those of Angel and Nina and if Buffy knew that I’m not sure how she would react. I take a stab at that in the story above as well.
Another fic that I think does an interesting job of dealing with Angel/Nina with Buffy is The Trouble We’re In which is a longer post NFA AU canon interpretation that goes off into some strange territories (including some Buffy/Spike and Angel/Spike slash stuff so beware!) BUT the Nina and Buffy dynamic in the fic, I find pretty interesting.