"She would make a good girlfriend."
Translation:Ella sería una buena novia.
It would be understandable without the article, but it is needed grammatically. You would not say "she would make good girlfriend".
"Amiga" and "novia" are very different things. When a man refers to his girlfriend as "amiga" he is either married to someone else or is saying that he doesn't expect the relationship to lead anywhere (friends with bebefits).
well, from my experience I can say that is better say 'sería' than 'haría', because it's indicating that a social role probably is going to be. (Ella sería una buena novia). Obviously, social roles can be made, but being (sería) is more often used (al least in my country). Sorry for my english, i'm not a native english speaker.