I'm tempted to say you can leave me out of it -- my God I know how touchy some bisexuals or at least wannabee bisexuals can be -- but since you've asked I do have some advice.
Bisexuality -- or what passes for same [more on this on another post] -- seems to be far more commonplace among women than men. Let's say for argument's sake that many of these women are truly, genuinely bisexual [especially those who are equally attracted to both sexes]. However, it stands to reason that a certain percentage, however large, are not bisexual but gay.
For instance. A bi-identified woman once said that she didn't like the term bisexual, but couldn't call herself a lesbian because [italics mine] once in a blue moon she was attracted to a man.
Of course "once in a blue moon" means "hardly ever," and if a woman is "hardly ever" attracted to a man, she's not bisexual, she's gay.
I know there are bisexual advocates who will howl to the moon over this. They think anyone who for any reason or on any occasion ever slept with both sexes is automatically bisexual. Therefore, my school boy/college day fumblings with the opposite sex make me bisexual. I don't think so.
Some of this is simply internalized homophobia [which some people try to deflect by claiming anyone who disagrees with them is "biphobic" -- throwing it all back onto the big, bad gay person, you see]. Some people -- like your girlfriend -- just don't want to be gay. The problem is they're attracted to their own sex and they just can't deal with it. Whatever small attraction they have to the opposite sex is blown entirely out of proportion and turned into genuine bisexuality, which it isn't. [Again, this is not to say there aren't genuine bisexuals.]
I believe there are many, many women who are in this self-hating category. [And don't get me started on the men!] Here's just a quick sample of genuine comments I came across on the Internet from some bi-identified women .
"I am more attracted to women than I am to men ... one woman I was with was the best sex ever..... but I am married to a man and very happy... but if I ever divorce I will never have another man I will go full lesbian..... women are the best!!"
Okay. I don't think I really need to comment on that, anyone over eight can read between the lines.
"I'm with a man now. I'm afraid of getting too emotionally close to women friends in case I fall in love with them."
This is another one that needs no comment.
The women who made these comments are not true bisexuals, they're very confused and conflicted lesbians, which is probably true of your friend. They don't even realize how their own words give them away. I've no doubt it's along the same lines with your friend.
The trouble is that some irresponsible bisexual advocates have made it impossible for these women to accept the truth about themselves, because -- as you discovered -- if you try to tell them they're not bisexual they'll tell you you're being "biphobic" -- and immediately end all discussion. [Frankly, I think in some ways it's a shame that some bisexuals think of themselves as a totally separate sexual minority. Gay people are not to blame for that.]
All you can do is keep trying in your own tactful way to get through to her. Don't insist that you're right, simply tell her what's on your mind, suggesting that she just possibly, conceivably, might be, could be a lesbian, and what the hell would be wrong with that? Point out to her that you're a good person, that you're happy being gay, that she's even said she envies the relationship you have with your lover. Suggest she get counseling.
If will be like walking on egg shell, but give it a try. If she becomes too implacable or nasty or just closes her ears to you with finality, if she becomes downright homophobic with you [not without precedent] well, then, give it up. You can help some people, but others just don't want to listen. You'll just have to hope that she'll be able to get past all the bisexual identity politics and internalized homophobia and work it out on her own.