A part of accepting someone is, like you said, accepting as a whole, which includes their past. However, that is all you have to accept it as; the past. Everyone will have a past in dating, regardless of how many partners they had or how well or badly they ended. If you wish to talk to your friend, you would need to suggest to him to sit down with his girlfriend and have them sort it out as to why she only resorted to calling him her "friend" instead boyfriend. I would be a little ticked too if someone whom I've been dating for a while only refers to me as a friend. 

Well I can understand being angry at the context. From looking in from the outside it seemed fishy, but then you also have to stop and think... maybe there is a reason she is using such vague comments... especially if they have the same friends and there was some major conflict in their relationship, maybe he was not a nice guy. I have had to be vague when speaking to some of my friends that I shared with my ex as he is a very obseessive person and has on numerous occassions tried to destroy my job, stalk me trying to find out who I was with, I actually caught him following me trying to find where I lived... I now no longer speak with people that knew my ex and I together as they would run and tell him everything I was doing, even though we had not been together for a long time. Just sometimes there are significant reasons for certain actions. Don't always jump to the wrong conclusion.

If there is not a good reason and she just wants to hide things that bodes ill for the relationship, but even if she were being vague to the person she was talking to on the phone, that doesn't mean you be vague with the person you are in a relationship with. Always be honest. While we don't always like hearing about past relationships, sometimes there are things that need to be discussed as with mine.

My friend just told me that his gf told him she actually DIDN'T think of him at the moment she talked on phone... and the "reason" she mentioned was she didn't want to see her ex (despite her current bf present or not)... to me, it looks like his gf was still interested in her ex....
Why?
my friend previously told me his gf still "want him to have a good life. Otherwise, what was my value? he HAS TO have a good life now!"....

say: "I'm sorry don't get me wrong or take this the wrong way, but are we bf and gf because the other day when talking on the phone you said I was a friend. I just want to know so that we are on the same page."
gah id be cheesed too.

Just let him know to get to the heart of the issue. Tell him to approach her and let her know how that made him feel, and to tell her to put herself in his shoes and see it from his eyes, as to why he got so upset. Ask her to try to be more considerate, but be as kind as possible in doing so. Otherwise, it may just result in pushing her away. While I understand him being upset, the anger thing needs to go. If he's going to get that upset, it's silly. Child like, you know? Instead, tell him to be respectful of the fact that she won't go to the party due to her ex being there. Let him know it's okay to be upset. Many would be. But don't be angry and silly about it. Just let her know how it made you feel, you know? But in a nice way. Yelling/screaming and or harsh tones and words never get the point across.